As a reference, measure using a string or ribbon around the circumference of the thumb. Measurements should be made wearing batting gloves if you play wearing them or not if you don't wear them.
L- 2 3/4" circumference (generally above 205+ lbs)
M- 2 1/2" circumference (circumference (generally 130-205 lbs)
S- 2 1/4" circumference (good for youth and adults under 130 lbs)
Use this as a guide. If in between sizes, we generally recommend going with the smaller size for a secure, snug, performance fit.