I bought the unstructured navy crepe blazer from a season ago. It was a size S and the pit-pit was 18", so a true 34. My chest is 36" so I need to let it out a little bit (which can probably barely be done), but the fit otherwise is great. So I definitely wouldn't recommend sizing down if my blazer is any indication of how they size in general.
11/11/12 at 7:48am