What can you do if your belly button opening ends up too small after a tummy tuck?

This can happen.When you re do the bellybutton it is a circular scar and as with all scars they contract.As it contracts then the opening gets smaller.This is easy to correct as an outpatinet under local.

If the belly button ends up appearing too small, it can be revised once the healing process is complete at about 6 months. If there is excess scarring that caused the opening to shrink, then excision of the scar tissue can be completed. If the opening was simply cut too small, then some small tissue rearrangements procedures can be used. Almost all options can be completed in the office with local anesthesia. 

I agree with Dr Burke.it is usually easier to make a small umbilicus (or belly button) larger than the other way around.Sometimes, you can have scarring or a stricture around the opening which may need to be released.See a board certified plastic surgeon to discuss your options.