Use it or lose it. It's difficult for me to 'use' it, but I try and achieve an erection at least 2-3 times a week to make sure nothing is going to atrophy. I'll probably be past 30 months of HRT before I can undergo SRS, so it makes sense to me that I need to keep whatever material I have in good condition. Too bad the 'maintenance' causes intense dysphoria. Either way, the ends justify the means to me.
So far at almost 11 months of HRT, I am under the impression that I may have had just a bit of shrinkage, but nothing very noticeable.