This shirt is technically a size M in a slim fit but I have lots of other shirts from Lulu and can say with confidence that this fits like a S. I am 5’10” 160 lbs and it is tight! Otherwise the shirt is really nice, light airy fabric that surprisingly doesn’t get smelly. Great for golf, business casual, or regular casual. 85% polyester, 10% Lyocell, 5% elastane.