I just got back from Disney and DC. Oh those wetsuits...
When we went up to the little hut where they are, this very cute, very nice 20 something year old (please keep in mind that I'm 24) was working. He didn't ask what sizes we were, but literally just glanced at us and handed us suits. He said "I think these will be fine but come back if they're not". I can't imagine a job where it's literally on you to size people up. He was very good about it though, I'm sure lesser people would look at someone heavier and roll their eyes or something rude but he was very polite and friendly about it. My friend's suit was fine and I had to go up a size but that was fine. The first time I went I was really scared of the whole thing but the girl that was working that time assured me they'd fit people way bigger than me and it wouldn't be a problem. I even came up with what I feel is the most efficient way to get into the suit. I call it the "double fisted butt shimmy". This works with the shorty suit. If you get the suit mostly up your legs, then stick your hands down the back of it and grab it, and simultaniously jump and shimmy while pulling the suit up, you can get it up over your hips. My friend favored something that resembled the twist crossed with the watusi. Either way, more often than not, they'll probably have something that will work for you. And yes, the provide quite a bit of bouyancy.