I am 5'6 and 135 lbs and bought a size medium. I usually wear a small in non-bib Pearl Izumi shorts and they are ever so slightly on the snug side, but I prefer that. For these bibs, I decided on a medium after reading other reviews. I'm glad I did, the fit is great. Maybe the P.R.O. sizing is smaller than my more budget friendly shorts?
I bought these for a 100 mile mountain bike race thinking that the bib style and thicker chamois would be nice for long days in the saddle. These have quickly become my favorite shorts. They are by far the most comfortable I own and I find myself grabbing them for 2-hour weeknight rides and long weekend days in the saddle. I also appreciate the drop tail construction for bathroom breaks.