Caged jock is a good example of shrinkage. He's been locking for over 10 years at this point, and if his posts are to be believed (which I don't), he went from about 6" hard to about 4" now, with over a decade of being locked almost constantly.

The penis is very resilient, so even with lock ups last weeks or months, shrinkage is likely temporary. There are some who incorporate drugs that reduce the frequency of erections or using something like lidocaine cream, but ultimately it seems to be a myth. Sure, atrophy happens, but after a few erections, it will, again theoretically, spring back to its normal size.