Yes, I have a couple Babykins onesies. I don't have the long-legs variety, though. Babykins definitely sizes their onesies to allow for thick cloth diapers. I even spoke with the owner on the phone who affirmed this fact--their onesies will probably be a bit baggy in the crotch if you wear disposables. I personally had my onesies shortened 2" because I mostly wear disposables; I do have a few thicker cloth diapers that I kind of wish I had unaltered onesies to wear with. It is a bit snug with thick cloth diapers--but the onesies stretch, so it's OK.
As for sizing... if you were wearing disposables, I'd say go for the medium simply because it will be shorter in the crotch than a large (due to you being 5'10.5" and the mediums only fit up to 5'8"). But since you are wearing cloth, it might be best to go with the large. I don't know how tight you like your shirts to be, but I have a 36" chest and ordered a small (38" chest). I think it fits great; perfect in the shoulders and good in the chest--not skin tight, but not ridiculously baggy. At a 39" chest, a large (42") would be 3" bigger in the chest than your actual measurement, which isn't a ridiculously huge difference. Of course, if you prefer your shirts to be very tight-fitting, it's not so great, but otherwise--it'll probably be fine, especially after you give it a few cycles in a hot wash & hot dryer.
Oh--and yeah, I wish they gave the collarbone to crotch measurement as well. I e-mailed to ask about that, but couldn't get a straight answer. So I just went with a small and ended up having to have them shortened after the fact. If you want to be sure, you can just give them your torso measurement and ask them to do something custom for you; be sure to let them know you plan to wear cloth diapers. It only costs an extra $5 per onesie for the alteration... Though that was for my non-leg onesies; the ones with legs might be harder to alter. Best luck; the Babykins onesies are super comfy.
