I am 5'9" and 135#, skinny athletic build. Always stuck between small and med, usually always get a small in base layer. First got small, I felt it fit my skinny body very well but the crotch was tight in my junk, who knows maybe it would have stretched out a bit. Returned and got the medium, looser fit throughout the body but fit my height well. From my experience rab has weird sizing. Wanted to give it four stars because of sizing but love the piece to much!
I love this baselayer because I travel a lot and tour. Therefore unlike other one pieces that are stinky after one day (maybe a couple days with wool) this one is good for tens of days, because it is farmer john build which doesn't go to underarms and when you wear underwear your stink is separated. It is also the most efficient weight to warmth with a farmer john. Therefore just throw a med base layer top and you are set. My most used piece of clothing in the winter. Also the material is very comfortable, warm, breaths, takes moisture very well.
So what's up with all these high end baselayers having the stupid chest pocket (this one does)?? For a while I go out of my way to buy without one (when I was comparing zip t from icebreaker to smartwool I leaned towards smartwool one of the reasons because didn't have one). Well I finally found a great use for that pocket. When stuck overnight in the backcountry I was able to use it to warm things up like my frozen orange, frozen phone so I could get 911 calls. Total life saver, so it can be used as a little oven other than that don't know any other purposeful uses.