diff --git a/tools/coordinate.py b/tools/coordinate.py
index 02f59bc..3609eff 100644
--- a/tools/coordinate.py
+++ b/tools/coordinate.py
@@ -195,10 +195,6 @@ class Coordinate:
 
 
 class Shape:
-    top_left: Coordinate
-    bottom_right: Coordinate
-    mode_3d: bool
-
     def __init__(self, top_left: Coordinate, bottom_right: Coordinate):
         """
         in 2D mode: top_left is the upper left corner and bottom_right the lower right
@@ -211,7 +207,7 @@ class Shape:
         self.bottom_right = bottom_right
         self.mode_3d = top_left.z is not None and bottom_right.z is not None
 
-    def size(self):
+    def __len__(self):
         if not self.mode_3d:
             return (self.bottom_right.x - self.top_left.x + 1) * (self.bottom_right.y - self.top_left.y + 1)
         else: