Thank you for your post, and I apologize for the late reply.
What can you tell me about the machine that is exhibiting the problem. That is, what version of Mac OS X? How much memory? What other applications are running? What version of FileMaker Pro 11? Do you have any special settings for your keyboard? When you go to print from any other application, does the Paper Size pop-up work similarly?
If you have found a solution in the mean time, please post here so others who encounter this behavior can benefit.