Remember to order 1-2 sizes down from what you're used to.  I'm 5'11", 175 lbs and normally wear a medium or large t-shirt.  The small size is a little snug in the chest but fits well everywhere else. If you order your normal size, Carhat's t-shirt will hang all over which I don't like. Shirts are somewhat heavy weight so best worn when the temps are on the cool side not on hot and humid days.