I'm 5'9 and 165 and bought Med. They fit tight, but not restrictive. The straps are great and lay flat. The coverage comes up high on the sides and about belly button height on the front, which suits me. The vertical leg grippers are awesome and do their job well, even if you're not a leg-shaver. The fabrics are thicker than usual, but they are quality. I think these would be hot to ride in above 80 or so.
The big disappointment is the pad. Yes, it's good for Castelli, but not a great value for the price. It floats on it's own panel which is kinda cool, but it isn't really better than bibs that don't have this design. The pad is dense, but for me, it's a 40 mile pad at best.
Was hoping for a suitable replacement for Body Paint bibs. These are close, but definitely not it. Every other Castelli bib shorts I have are size Large. So are these, but the shoulder straps are much too short and the leg openings are too small (when compared to the other Castelli bibs.) I have them...I'll wear them...But, they're not my favorites.
This is the best Castelli yet (Ive had many over the years). I cycle 10k miles per year going on two decades. These shorts are compressive, keep everything comfortably in place, yet just disappear. I find the padding to be outstanding and not at all bulky. I found these to be so good I immediately purchased a second pair and look forward to retiring all the rest.