I can fit my son (who wears mediums in most everything, 12" thighs, 17" waist, 16" rise) into a size small fuz easy on the last snap settings, although it's gotta be a trim diaper underneath! :LOL Funnily enough, he wears a size medium on the next-to-last snap settings, it just has way more rise, and is a little bit poufier fit on him.
For my babe-on-the-way, I have xs fuz easysand they fit great over nb/sm KHW fitteds (on my teddybear), but I can see them not fitting once the thighs get past 10" or the rise gets longer than 13.5"-14", whereas my nb Kiwis will fit a looong time. If you want a cover for a new baby, get the small, and it will fit a good long while, until babe perhaps becomes such a chunker around 4-8 months(before becoming mobile) and has 13" thighs or a 19" waist or something like that (my son was that size for a few months between 4-9 mo and a small easy would not have fit--but then it would have started fitting again later).
For my babe-on-the-way, I have xs fuz easys