Please see photos on how to measure for the below sizes-

XS- 2 9/16”, 6.1-6.9cm

S- 2 7/8”, 6.9-7.9cm

M- 3 3/8”, 7.9-9.0cm

L- 3 ¾”, 9.0-10.1cm

XL- 4 ¼”, 10.1-11.4cm