I'm usually M, except in button-ups where i have a hard time fitting my chest, but L things are always too big in the body. I read other reviews and decided to go with L for this shirt. It was soo perfect! It hugged all the right places, with sun-shirt athletic-wear stretch and breathability. I wore it to a show and it was so easy to pull on and off in the tiny tack area in the trailer. Collar stood nicely, and I'll feel good wearing it for open shows, ranch and western dressage. I want more colors now! (5'6", 38D, 175lbs)