In reply to Mr Fuller:
I think just following sizing used in other clothing 'genres' might be sensible, rather than the niche sizing that confuses everyone else.
I picked up a pair of Craft bib shorts in TKMaxx at the weekend*. They were sized men's XS, and I knew they wouldn't fit. But they will only fit a child. Fortunately, one of my colleagues has very keen cyclist children, so it may go her way, even if only for training use; they had a half-decent pad.
* I bought them because they were reduced, multiple times, and were only £1. So it would have been rude not to. Claimed RRP was a laughable £130. £30 more like...