I'd love to, but I'm a newbie and actually don't know how to do that, nor am I 100% sure what a capx is lol
When you edit your object, lowest Icon in editor, below image points.
That box defines with what area will your object interact with other objects or mouse in this case.
Sometimes you will want to make it larger than object, sometimes, smaller.
Take into account, more points on collision box means more hardware requirements.