NFL, NHL, MLB, NBA use different sizing charts (epecially Nike and Reebok NFL jersey) . It is more important to look at the absolute numbers instead of the "Lettering" like Size M / L / XL. At the "Size Selection" column in our listings, there will be a numerical number in round bracket. This refers to the chest size in inches.
Chest size= Armpit-Armpit length X 2
As a quick guide, you may want to measure your T-Shirt as reference. When you use T-Shirt measurement as estimate, you may want to give more allowance as jerseys are more comfortable when worn loosely