My chest is 36D, and I have the Underworks 983. Works like a dream. It doesn't give me a chest that you could play cricket on, but it looks more like a natural guy chest, if you get me. Love the thing. Unfortunately, my moobs swell and get extremely tender that time of the month, and I can't even get it on, it hurts too much. If you're like me, maybe think of getting another one with velcro.
I got a medium, fits great. A small wouldn't fit me, for sure, so don't try going smaller for a tighter bind. Not that you'll need to. I try to really limit the length of time I wear it, but I wore it out the other weekend up to Leeds, which in total ended up being a 15-hour day. Wore it the whole time, walking around, on the train, running to make connections... I was hot, but not terribly so. Nor does it affect my breathing, though I do feel a bit lightheaded sometimes. I suspect that's more getting used to it and having just quit smoking while not drinking much water on active sunny days. I think the general advice is no more than eight hours, though. I usually wear mine out, then take it off to give them a break when I get home (and with the temperature in Switzerland starting in the thirties now, I have to). Sooooo... go nuts.