Take a loose tape measure and wrap around your wrist. Keep in mind that if you plan to wear several pieces at a time you may want to pick a size up for the ones that will be worn higher on your arm.    

Woman

-small: very petite wrists, 6.0" or smaller

-medium: measures 6.5-7" (average woman's wrist size)

-large: measures 7.5"+

Male

-small: 7.5" or smaller

-medium: 8" (most common in men's wrist size)

-large: 8.5"+

Tweens

-small: 5.5"

-medium: 6"

-large: 6.5"+