Add my vote for Small to the tally. I think it has the right amount of fluid drape to it for your frame, where it's very figure flattering. The Medium could be do-able depending on how you styled it, but the skimming fit of the small I think will make it so you can style this one in more ways (more dressy OR casual, whereas the Medium's degree of slouch looks so casual that I think it would work less well if you wanted to glam it up, imho).
And props to you for choosing a jumpsuit! It's one of those pieces that I love the way they look but I personally just won't buy because I can't stand the whole "have to undress just to use the bathroom" situation!
