The shirt's form is sized according to the relative form and standards of Vietnamese people. If you are considering between two sizes, you should choose the larger size.
Size S: Height under 1m65, weight under 55kg
Size M: Height from 1m65 - 1m75, weight under 65kg
Size L: Height from 1m75 - 1m85, weight under 90kg
Size XL: Height from 1m85 or more, weight under 120kg
