SizeConstraint (Enum)

If you have a square on the screen, each side is 10 pixels wide. Then you make the horizontal size of your 3D view twice as wide. The SizeConstraint Enum has 3 numbers:
Enum	Name	Description
0	RelativeXY	With the SizeConstraint set to RelativeXY, the box will resize itself to match this width. Therefore, it will now be 10 pixels high, but it will also be 20 pixels wide.
1	RelativeXX	With the SizeConstraint set to RelativeXX, the box will resize itself to match only the change in width of the 3D view. Your box will stay a square, but it will change size.
2	RelativeYY	With the SizeConstraint set to RelativeYY, the box will resize itself to match only the change in height of the 3D view. Your box will stay a square, but it will change size.