I draw a square in Panini, and convert it to a button. Then I want to set its size to 15x15. However, the minimum of width and height in Panini is 18x18. I need to activate its 'cover' 'down' 'disable' status first, then I can set the size to 15x15. In CS5, it is a different story. I think it should be a bug.
Have you made sure the minimum size of the button skin is smaller than 15px in all states?
If you have a rectangle inside of a button skin with no constraints, you will not be able to resize your button smaller than the size of the rectangle. If your rectangle has a fixed size, then the minimum size of the button will be the same as the size of your rectangle.
If you wish to make your button resize, you will probably need to constrain it on all sides in all states.