Sizing ? I have worn vests for years and prefer a vest if i can get by with it for extra warmth when i am Moving. I wear them X-country skiing, hiking, Choring (cutting wood, feeding animals, ect), Framing, and I love my Wooltimate vest for cruising through Elk country during the archery season. I wear an XL top comfortably, if it has sleeves. My vests are L, and I had to downsize to M for my Wooltimate hunting vest. I usually have an undershirt / baselayer and sweatshirt or something similar under my vest. I think you want a little tighter fit with a vest to trap more of the body heat, and you don't have to worry about the sleeves and shoulders binding. You can always put a shell or layer over. My experience with vests for what it is worth.