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.
I reserve these for longer rides, where the advantage over less expensive options becomes more obvious. I like the feel of the material, and the straps and leg cuffs are very comfortable. However, what impresses me most are two things. First is the chamois, which is great because I don't notice it or think about it (unlike some others that make me squirm around or get out of the saddle every so often after 20-25 miles). I was expecting that from other reviews I've read, but what I didn't expect and really like is the slight compression the bib shorts provide. I have other bib shorts that feel snug and support my muscles, but the Aero RC bib shorts go beyond that and seemingly help my legs stay fresh longer. Size up as usual for Castelli and take them out for a very long spin. Yeah, you'll still wish they didn't cost so much, but you'll understand why they're worth it.
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.
Very nice socks. Italian made and they both look and feel great. They are a bit snug (read "aero"), which I suppose is the point since the UCI limits the length pro riders can wear in races. I wear a size 42 Fizik shoe, and I ordered the L/XL size sock. I was really surprised how tight these socks fit on my smaller, narrow feet - so be warned to order a larger size if you're a bigfoot. Overall I am very happy with these socks and will definitely order more pairs as I need them.