diff --git a/src/painter.py b/src/painter.py
index f11cfe8..f91558e 100644
--- a/src/painter.py
+++ b/src/painter.py
@@ -43,6 +43,9 @@ class Painter(Widget):
         # modifiers
         self.modifiers=[]
 
+        # base size for all particles
+        self.base_size=50
+
         # init Widget
         super(Painter,self).__init__(**kwargs)
 
@@ -75,13 +78,13 @@ class Painter(Widget):
 
     # convert logical coordinates (normalized and centered) to the ones that are plotted
     def pos_tocoord_x(self,x):
-        return self.width/2+x*Square_element.size
+        return self.width/2+x*self.base_size
     def pos_tocoord_y(self,y):
-        return self.height/2+y*Square_element.size
+        return self.height/2+y*self.base_size
     def coord_topos_x(self,x):
-        return (x-self.width/2)/Square_element.size
+        return (x-self.width/2)/self.base_size
     def coord_topos_y(self,y):
-        return (y-self.height/2)/Square_element.size
+        return (y-self.height/2)/self.base_size
         
 
 
@@ -176,10 +179,10 @@ class Painter(Widget):
             # zoom
             elif text=="+":
                 # increment by 10%
-                self.set_zoom(Square_element.size/50*1.1)
+                self.set_zoom(self.base_size/50*1.1)
             elif text=="-":
                 # decrease by 10%
-                self.set_zoom(Square_element.size/50*0.9)
+                self.set_zoom(self.base_size/50*0.9)
             elif text=="=":
                 # reset
                 self.set_zoom(1)
@@ -557,7 +560,7 @@ class Painter(Widget):
 
     # set zoom level
     def set_zoom(self,level):
-        Square_element.size=level*50
+        self.base_size=level*50
         self.draw()
 
 
diff --git a/src/polyomino.py b/src/polyomino.py
index 6a79106..ed82f0d 100644
--- a/src/polyomino.py
+++ b/src/polyomino.py
@@ -29,7 +29,7 @@ class Polyomino():
             Color(r/2,g/2,b/2,alpha)
 
         for square in self.squares:
-            Rectangle(pos=(painter.pos_tocoord_x(square.pos.x-0.5),painter.pos_tocoord_y(square.pos.y-0.5)),size=(square.size,square.size))
+            Rectangle(pos=(painter.pos_tocoord_x(square.pos.x-0.5),painter.pos_tocoord_y(square.pos.y-0.5)),size=(square.size*painter.base_size,square.size*painter.base_size))
 
         # draw boundary
         self.stroke(painter)
@@ -44,11 +44,11 @@ class Polyomino():
         Color(1,1,1)
         for square in self.squares:
             Line(points=(
-            *(coordx-0.5*square.size,coordy-0.5*square.size),
-            *(coordx-0.5*square.size,coordy+0.5*square.size),
-            *(coordx+0.5*square.size,coordy+0.5*square.size),
-            *(coordx+0.5*square.size,coordy-0.5*square.size),
-            *(coordx-0.5*square.size,coordy-0.5*square.size)
+            *(coordx-0.5*square.size*painter.base_size,coordy-0.5*square.size*painter.base_size),
+            *(coordx-0.5*square.size*painter.base_size,coordy+0.5*square.size*painter.base_size),
+            *(coordx+0.5*square.size*painter.base_size,coordy+0.5*square.size*painter.base_size),
+            *(coordx+0.5*square.size*painter.base_size,coordy-0.5*square.size*painter.base_size),
+            *(coordx-0.5*square.size*painter.base_size,coordy-0.5*square.size*painter.base_size)
             ))
 
     # move by delta
@@ -74,17 +74,17 @@ class Polyomino():
 # square
 class Square(Polyomino):
     def __init__(self,x,y,**kwargs):
-        super(Square,self).__init__(**kwargs,squares=[Square_element(x,y)])
+        super(Square,self).__init__(**kwargs,squares=[Square_element(x,y,size=kwargs.get("size",1.0))])
 
 # cross
 class Cross(Polyomino):
     def __init__(self,x,y,**kwargs):
         super(Cross,self).__init__(**kwargs,squares=[\
-        Square_element(x,y),\
-        Square_element(x+1,y),\
-        Square_element(x-1,y),\
-        Square_element(x,y+1),\
-        Square_element(x,y-1)\
+        Square_element(x,y,1),\
+        Square_element(x+1,y,1),\
+        Square_element(x-1,y,1),\
+        Square_element(x,y+1,1),\
+        Square_element(x,y-1,1)\
         ])
 
     # redefine stroke to avoid lines between touching squares
@@ -95,30 +95,29 @@ class Cross(Polyomino):
 
         Color(1,1,1)
         Line(points=(
-        *(coordx-0.5*Square_element.size,coordy-0.5*Square_element.size),
-        *(coordx-0.5*Square_element.size,coordy-1.5*Square_element.size),
-        *(coordx+0.5*Square_element.size,coordy-1.5*Square_element.size),
-        *(coordx+0.5*Square_element.size,coordy-0.5*Square_element.size),
-        *(coordx+1.5*Square_element.size,coordy-0.5*Square_element.size),
-        *(coordx+1.5*Square_element.size,coordy+0.5*Square_element.size),
-        *(coordx+0.5*Square_element.size,coordy+0.5*Square_element.size),
-        *(coordx+0.5*Square_element.size,coordy+1.5*Square_element.size),
-        *(coordx-0.5*Square_element.size,coordy+1.5*Square_element.size),
-        *(coordx-0.5*Square_element.size,coordy+0.5*Square_element.size),
-        *(coordx-1.5*Square_element.size,coordy+0.5*Square_element.size),
-        *(coordx-1.5*Square_element.size,coordy-0.5*Square_element.size),
-        *(coordx-0.5*Square_element.size,coordy-0.5*Square_element.size),
+        *(coordx-0.5*painter.base_size,coordy-0.5*painter.base_size),
+        *(coordx-0.5*painter.base_size,coordy-1.5*painter.base_size),
+        *(coordx+0.5*painter.base_size,coordy-1.5*painter.base_size),
+        *(coordx+0.5*painter.base_size,coordy-0.5*painter.base_size),
+        *(coordx+1.5*painter.base_size,coordy-0.5*painter.base_size),
+        *(coordx+1.5*painter.base_size,coordy+0.5*painter.base_size),
+        *(coordx+0.5*painter.base_size,coordy+0.5*painter.base_size),
+        *(coordx+0.5*painter.base_size,coordy+1.5*painter.base_size),
+        *(coordx-0.5*painter.base_size,coordy+1.5*painter.base_size),
+        *(coordx-0.5*painter.base_size,coordy+0.5*painter.base_size),
+        *(coordx-1.5*painter.base_size,coordy+0.5*painter.base_size),
+        *(coordx-1.5*painter.base_size,coordy-0.5*painter.base_size),
+        *(coordx-0.5*painter.base_size,coordy-0.5*painter.base_size),
         ))
 
 
 
 # square building block of polyominos
 class Square_element():
-    # size
-    size=50
 
-    def __init__(self,x,y,**kwargs):
+    def __init__(self,x,y,size,**kwargs):
         self.pos=Point(x,y)
+        self.size=size
 
     # set position
     def setpos(self,x,y):
@@ -128,12 +127,12 @@ class Square_element():
 
     # check whether a square at pos interacts with square
     def check_interaction(self,pos):
-        return l_infinity(pos-self.pos)<1
+        return l_infinity(pos-self.pos)<self.size
 
     # check whether a square at position pos is touching self
     def check_touch(self,pos):
         # allow for error
-        if in_interval(l_infinity(pos-self.pos),1-1e-11,1+1e-11):
+        if in_interval(l_infinity(pos-self.pos),self.size-1e-11,self.size+1e-11):
                 return True
         return False
 
@@ -143,21 +142,21 @@ class Square_element():
         if v.x!=0:
             if v.y!=0:
                 intersections=[\
-                Point(self.pos.x+1,pos.y+v.y/v.x*(self.pos.x+1-pos.x)),\
-                Point(self.pos.x-1,pos.y+v.y/v.x*(self.pos.x-1-pos.x)),\
-                Point(pos.x+v.x/v.y*(self.pos.y+1-pos.y),self.pos.y+1),\
-                Point(pos.x+v.x/v.y*(self.pos.y-1-pos.y),self.pos.y-1)\
+                Point(self.pos.x+self.size,pos.y+v.y/v.x*(self.pos.x+self.size-pos.x)),\
+                Point(self.pos.x-self.size,pos.y+v.y/v.x*(self.pos.x-self.size-pos.x)),\
+                Point(pos.x+v.x/v.y*(self.pos.y+self.size-pos.y),self.pos.y+self.size),\
+                Point(pos.x+v.x/v.y*(self.pos.y-self.size-pos.y),self.pos.y-self.size)\
                 ]
             else:
                 intersections=[\
-                Point(self.pos.x+1,pos.y+v.y/v.x*(self.pos.x+1-pos.x)),\
-                Point(self.pos.x-1,pos.y+v.y/v.x*(self.pos.x-1-pos.x))
+                Point(self.pos.x+self.size,pos.y+v.y/v.x*(self.pos.x+self.size-pos.x)),\
+                Point(self.pos.x-self.size,pos.y+v.y/v.x*(self.pos.x-self.size-pos.x))
                 ]
         else:
             if v.y!=0:
                 intersections=[\
-                Point(pos.x+v.x/v.y*(self.pos.y+1-pos.y),self.pos.y+1),\
-                Point(pos.x+v.x/v.y*(self.pos.y-1-pos.y),self.pos.y-1)\
+                Point(pos.x+v.x/v.y*(self.pos.y+self.size-pos.y),self.pos.y+self.size),\
+                Point(pos.x+v.x/v.y*(self.pos.y-self.size-pos.y),self.pos.y-self.size)\
                 ]
             else:
                 print("error: move_on_line_to_stick called with v=0, please file a bug report with the developer",file=sys.stderr)
@@ -168,7 +167,7 @@ class Square_element():
         dist=math.inf
         for i in range(0,len(intersections)):
             # check that it is on square
-            if abs(intersections[i].x-self.pos.x)<=1+1e-11 and abs(intersections[i].y-self.pos.y)<=1+1e-11:
+            if abs(intersections[i].x-self.pos.x)<=self.size+1e-11 and abs(intersections[i].y-self.pos.y)<=self.size+1e-11:
                 if (intersections[i]-pos)**2<dist:
                     closest=intersections[i]
                     dist=(intersections[i]-pos)**2
@@ -184,9 +183,9 @@ class Square_element():
     def move_along(self,delta,pos):
         rel=pos-self.pos
         # check if the particle is stuck in the x direction
-        if isint_nonzero(rel.x):
+        if isint_nonzero(rel.x/self.size):
             # check y direction
-            if isint_nonzero(rel.y):
+            if isint_nonzero(rel.y/self.size):
                 # in corner
                 if sgn(delta.y)==-sgn(rel.y):
                     # stuck in x direction
@@ -199,7 +198,7 @@ class Square_element():
             else:
                 # stuck in x direction
                 return self.move_stuck_x(delta,pos)
-        elif isint_nonzero(rel.y):
+        elif isint_nonzero(rel.y/self.size):
             # stuck in y direction
             return self.move_stuck_y(delta,pos)
         # this should never happen
@@ -213,11 +212,11 @@ class Square_element():
         # do not move past corners
         rel=pos.y-self.pos.y
         if delta.y>0:
-            if rel<math.ceil(rel)-1e-11 and delta.y+rel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0:
+            if rel<math.ceil(rel/self.size)*self.size-1e-11 and delta.y+rel>math.ceil(rel/self.size)*self.size+1e-11 and math.ceil(rel/self.size)*self.size!=0:
                 # stick to corner
-                candidate.y=math.ceil(rel)+self.pos.y-pos.y
+                candidate.y=math.ceil(rel/self.size)*self.size+self.pos.y-pos.y
         else:
-            if rel>math.floor(rel)+1e-11 and delta.y+rel<math.floor(rel)-1e-11 and math.floor(rel)!=0:
+            if rel>math.floor(rel/self.size)*self.size+1e-11 and delta.y+rel<math.floor(rel/self.size)*self.size-1e-11 and math.floor(rel/self.size)*self.size!=0:
                 # stick to corner
                 candidate.y=math.floor(rel)+self.pos.y-pos.y
         return candidate
@@ -228,12 +227,12 @@ class Square_element():
         # do not move past corners
         rel=pos.x-self.pos.x
         if delta.x>0:
-            if rel<math.ceil(rel)-1e-11 and delta.x+rel>math.ceil(rel)+1e-11 and math.ceil(rel)!=0:
+            if rel<math.ceil(rel/self.size)*self.size-1e-11 and delta.x+rel>math.ceil(rel/self.size)*self.size+1e-11 and math.ceil(rel/self.size)*self.size!=0:
                 # stick to corner
-                candidate.x=math.ceil(rel)+self.pos.x-pos.x
+                candidate.x=math.ceil(rel/self.size)*self.size+self.pos.x-pos.x
         else:
-            if rel>math.floor(rel)+1e-11 and delta.x+rel<math.floor(rel)-1e-11 and math.floor(rel)!=0:
+            if rel>math.floor(rel/self.size)*self.size+1e-11 and delta.x+rel<math.floor(rel/self.size)*self.size-1e-11 and math.floor(rel/self.size)*self.size!=0:
                 # stick to corner
-                candidate.x=math.floor(rel)+self.pos.x-pos.x
+                candidate.x=math.floor(rel/self.size)*self.size+self.pos.x-pos.x
         return candidate