I guess you could say just a hair smaller than M. I got the M size and it fits fine, not exactly form fitting but close. I can wear a sweater under her. The neck is very snug. I would not say the jacket is too stiff. I live in the windy city and it is great for knocking down wind. It also keeps you dry. I walked for about 5 min. in soaking rain to find my torso still dry. I would recommend the product. The jacket is a little short and comes only to the belt line, that is my only complaint.
I think it is apt to point out that most complaints about the sizing of the jacket are probably due to negligence on the part of the buyer. I went and tried on the different sizes before I bought it at a smashing price, it fits fine. The tight neck is important when it is raining, it keeps the rainwater from soaking you from the top.