According to our statistics:
Woman : 71% Small - 26% Medium - 3% Large
Man : 23% Small - 59% Medium - 18% Large

Wrap a tape measure once around your wrist to observe the result. The tape measure must adhere to the wrist.

Your wrist measures from 15 to 16.5cm choose the size Small
Your wrist measures >16.5 to 18.5cm choose size Medium
Your wrist measures >18.5 to 20cm choose size Large