I, too, am usually a S or 6 in street clothes, (and a 34B up top). I have two Decente jerseys in M, and they fit just right. Not tight in the shoulders, long enough in back. I have a long-sleeve that's fine up front, but I feel like my summer jersey is a bit short. Maybe my shorts (which are also Descente) come up lower than my tights do, I don't know. It's not really a problem while riding, but if you get off the bike you may want to give the front a tug to make sure your stomach's not showing I definitely wouldn't feel comfortable in a S. Hope it fits you, overall I've been happy with mine.