I can wear a small one as well but I won't be able to move... keep in mind what you will be wearing under it. You can always sinch it down a bit if you need to however making it open further is a bit hard to do.
United States of America/N.Y.C./WEMIA/ commeth the hour, not commeth the erection
Measure your chest, and then get the size set for your chest. If your chest is bigger then 37 inches, a meduim should be your size. A large may fit but will be to big after you load gear on it and it will sag..