I usually wear a 15.5, and a Small fits my neck comfortably. For what it's worth, I am 6'1 with long arms (I usually take 15-15.5/34) and the small sports shirts are the best-fitting OTR shirt that I've found (37" chest/155 lbs). I'll try to post fit pics at some point. I picked up three and I'm very satisfied with them. I don't believe this. I am 5'7, 35" chest, 140 lbs. The size small extra slim sport shirt is very snug fitting, not skin tight, but I cannot move my arms around easily.