Measure your wrist tightly using a measuring tape.
Onesize varies from 17-21cm and fits both women and men. 
Find your fit for each collections:
GEMSTONE COLLECTION
Size Wrist Circumference (cm)  Wrist circumference (inch)
Onesize 17-21 6.7-8.3
 
SPEEDCUFF COLLECTION
Size Wrist Circumference (cm)  Wrist circumference (inch)
Onesize 17-21 6.7-8.3
 
LEATHER ESSENTIALS
Size Wrist Circumference (cm)  Wrist circumference (inch)
19 18-19 7.1-7.5
20.5 19.5-20.5 7.7-8.1