I have two of them in boys XL (18-20) Husky. They fit me and I am 5' 10" and around 210lbs. The only other thing I have that I can compare them too would be a Rearz body suit, I have one in large and if you lay them on top each other they are almost the same size, the Thereabouts is actually a little longer but the material it's made of is not as stretchy so in practice it is a little snugger through the crotch. around the chest is also a little smaller than the Rearz so the Thereabouts 18-20 Husky is probably like an adult medium.
Even the size I got still only has 3 snaps for the crotch and it is not as wide as the Rearz so a little more sides and leg of a diaper would show around the legs of the body suit.
I got them planing on using them when out in public because of their two layer design (the onesie inside a t-shirt) the one problem I have is the patches where the holes for a feeding tube are leave an imprint that you can see (because of how snug it is on me a skinnier person might not have that problem), this is very minor and I doubt anyone would notice unless they were staring at you for a long time. Its more like when you first where a diaper in public and you feel like it is super loud and every one will hear it type of problem. That being said I wouldn't wear it around people I know but going shopping stuff that I would. Also the two I have are the plain green one and the red one with the game controller, and the problem with seeing the patches is only on the plain one, on the printed one the vinyl makes the material stiffer and hides the patches..
For what they cost I would recommend them, If you just want them for around the house the one issue is not a problem anyway.