I have a week's worth of BabyPantz onesies and have found, unfortunately, that they run about two sizes too large. I'm not sure who designed them, but, they must think us big American babies are GIANTS! They do leave quite a bit more room than is necessary, even under the guise of "for bulky cloth diapers", and no amount of hot washing and drying will shrink them to the right size.
Being typically a size Large in most men's t-shirts, I ordered a medium my first time around. I now have three mediums that I can't wear beacuse they're too big. (Anyone interested in some lightly used onesies? Great for you XL boys out there!) I dropped down to Small, which I use for daily wear. I just receieved an XS as an xmas gift and it fits PERFECT.
Cliff's Notes for you all: Order two to three sizes down.