you would with any cuff bracelets. if it's a good fit, then your bracelet size is medium. if you find it very loose around your wrist, you're probably a small. but if the fit is too tight*, you should probably go with the large.
*actual metal bracelets are not as flexible as this plastic size guide, so if you find yourself flexing the size guide to get it around your wrist, you should probably go with the large size. 