Based on shoe size.

SIZE MEN WOMEN
SMALL 4 - 6.5 (EUR 35-38) 5 - 7.5 (EUR 35-38)
MEDIUM 7 - 9.5 (EUR 39-42)
8 - 10.5 (EUR 39-42)
LARGE
10-12.5 (EUR 43-46)
11-13 (EUR 43-46)
X-LARGE 13+ (EUR 46+)
 13+ (EUR 46+)