Everyone reacts differently. Chrissie has reported significant reduction in male function, I believe including shrinkage, but she has been using PM for over 3 years. I think most others (not using PM as long) have had similar experiences to Bryony. I certainly have.
This is how I understand male function, spontaneous erections, and possible shrinkage when those go away. It's almost like the male system is constantly practicing erections spontaneously to keep the tissues involved stretchable and thus able to achieve erection when the real deal happens. On PM (which seems to be antagonistic to testosterone if it doesn't actually reduce T production) most have noticed spontaneous erections pretty much disappear including the typical morning 'wood.' I think the consensus is that erections are still possible with proper stimulation, it's just you aren't as interested in doing that and they stop happening on their own.
(
chrissie, don't read any further; no coronaries please!) If you want to maintain your size/length you basically need to "exercise" the tissue in question by having regular erections. If you don't do this regularly you will probably notice shrinkage of your member. I know of several MtF women that pump regularly with the idea of maintaining enough tissue for a deeper neo-vagina once they have SRS. So, you don't actually have to have sex or masturbate to keep things stretched out if either of those activities bothers you.
As far as the testicles go, I suppose those would shrink with long-term use (really disuse!), again because the PM seems to be antagonistic to the effects of testosterone. In my experience, I don't even need an anti-androgen while taking PM. If you are taking SP (which is supposed to block the conversion of T to the more potent DHT) you might get even more shrinkage. I think mine are smaller now and I've been doing NBE for about a 18 months, taking PM for over a year of that. I am older, already sterile, and don't desire anymore children so that's not a problem for me. A smaller package fits better in size 10 jeans anyway!
It would be nice to hear the experiences of some of the younger folks.