I'm 33" at the biggest part, 29" ribcage, and I fit into an Underworks small, a medium is definitely too big on me, though much easier to get on. But the biggest issue I have is getting it over the shoulders or hips when getting it on - if you've got small hips, you might want to try it and see. Can always resell it to the next guy.
If the poster above doesn't want it, I'll take it off your hands, and I'm in Canada.