This hat has M2 performance fabric that wicks moisture and heat away.  It has an elastic comfort fit sweatband inside for a great fit.  Measurements for the sizes are below.

S-M=  55-59 centimeters, 21 5/8-23 1/8 inches, or size 6 7/8-7 3/8

M-XL= 59-64 centimeters, 23 1/8-25 1/8 inches, or size 7 3/8- 8