I use them for work, I'm a metal worker and sometimes I work outside in the bitter winter cold. Sizes, I have Medium and L. The material is very stretchy and even if you order a size down it will most likely fit you after a few uses and a good wash in cold water. I've been wearing it in 10-15°F weather for 6-8 hours and I can say it will keep you warm no matter what, I actually feel a bit sweaty if I move too much while wearing it wear under my work clothes! The material is very thin, but as Under Armor always does, it delivers every time you need to generate/store body heat! This is a very good product, made from the highest quality materials (which is what most UA products are made of), plus I have several UA sweatshirts with the same thermal properties, thin material design and thoroughly enjoy each one! 5 Stars!