By the circumference of the fullest part of the forearm, usually right below the elbow.
Extra Small: Fits 5.0 - 6.5 inches, weighs 1/4 lb.
Small: Fits 6.5 - 8.0 inches, weighs 1/2 lb.
Medium: Fits 8.0 - 9.5 inches, weighs 1 lb.
Large: Fits 9.5 - 11.5 inches, weighs 1 lb.