There are multiple possible solutions, but one approach is to pass a reference to the checkbox into the CreationTool's constructor, then store it as an instance variable, so that inside of the mouse click handling method you then have a reference to the checkbox and so you can check if the checkbox is selected.
If it is, you can modify the new figure to be twice as big (before it gets added to the Drawing).
(Note: a similar approach can be used for choosing a Color for the next figure that gets created.)
