I am 5' 8" and a pretty fit 175 lbs or so (not a super lean bicyclists build but not fat either), I bought an XL and it fits great. My absolute favorite thing about these bibshorts is the Kiss Air2 pad, it is so comfy, I wish I would've purchased these years ago. Much comfier than the Pearl Izumi bottoms I've been riding for the last 10 or so year. I've been riding these shorts in HOT weather and find they are pretty breezy without showing everyone else your stuff. I also read where another reviewer had a painful experience due to the location of the straps. I fortunately have not experience this, but the review did cause me to notice the placement of the straps on these shorts is more inboard than the Velocissimo bibtights that I also really like. I'll be buying more Castelli bottoms because of the comfort factor.
First, the elephant in the room... sizing. Conventional wisdom is that Castelli runs small and that you should size up, but let me offer an alternative--read the size chart! I purchased the size indicated according to the measurements in the size chart and these fit perfectly. As to the bibs themselves, the chamois is super comfortable and I really like the compression and lightweight fabric. Longest ride in these so far is 100km, and they could have gone further. Really the only problem is that now I want to upgrade my old entry-level bibs because these are so much better!
I wanted to try a new brand of bib and I was not disappointed. The fit is tight, but it is a race bib. I based my size off of other brands I wear and I might have gotten the next size up. That said, the bib doesn't bind, give solid compression, and is cool for the Florida summer
I have learned that buying something of the quality of Castelli is the way to go if you ride a lot. I love these shorts the fit is perfect for me. I am 160 pounds and 5'8" a medium is a good snug fit.
Loved these. Recommended by a friend. Road 545 miles in 10-days during an event and they held up great! Hand washed every night (in clothing bags, and hang dried) and they feel brand new.