This is an interesting question. This is possible for iParts, but I don't know of a way to do it with user parameters. It could be handled with a little iLogic programming. Are you familiar with iLogic?
I certainly wouldn't know how, but you can assign a multivalue list to the parameter which only includes the allowed sizes. Right-click the parameter and choose make multi-value. Type in the allowed values and as you choose them they will update that dimension.
Steven White Lee C. Moore, Inc. www.lcm-wci.com Inventor 2011 Intel Dual Xeon E31225 @ 3.1 GHz CPU 16 GB RAM NVIDIA Quadro 600 GPU Windows 7 - 64 Bit