These great T shirts are made of Coolmax. Coolmax is a special absorbent fabric that allows you to stay sweat-free and cool in the hot weather without clinging to your skin and also helps you retain heat in the cooler climate as well. What's best about them is that the lightweight material also dries at a rapid pace after washing so they'll be ready to wear in no time. 90% Cotton, 10% Coolmax. Please advise size.