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. I mainly bought these for cool, wet mountain biking due to a very rainy spring this year AND to try out Castelli bottoms. I typically wear these when it's 40-60 degrees out and they are just fine, I think I could easily, comfortably ride these in to the 30's (and plan to do so this fall). My absolute favorite thing about these bibtights tho is the Kiss Air2 pad, it is so comfy, I wish I would've purchased these years ago. I have been riding Pearl Izumi bottoms for a decade and they are so-so comfy wise, I just thought they were the industry standard so I didn't think there was better gear out there. Anyhow, here's where the Castellis are so comfy: I live in a valley and most of the fun MTB rides are an hour(s) long climbs on logging roads. With the PI gear I start to get a buttache about 1/2 hour in, but with the Castelli gear I can climb a couple of hours without being forced to constantly change my seating position or stand up on the pedals just to give my bum a break. I've already bought some more Castelli bottoms and they are also very good.
Warm, comfy, good stretch, price is okay, nice chamois, look good, mellow branding, appropriate reflection, easy to put on despite no zippers at leg opening. Haven't ridden them in rain yet but previous Nanoflex items worked well in moderate precipitation. Bonus: if you're a socks-on-the-outside type no zipper = no chafing.