Small: 6.5 inches. Usually fits younger people and the average woman with thin wrists.
Medium: 7 inches. Usually fits the average woman, men with thinner wrists, and teenage boys.
Large: 7.5 inches. Usually fits the average man.
When measuring your wrist, stay as close to your hand as possible, like so (the circumference in the picture is 6 inches.)
If you don't have a measuring tape, use a piece of string or something you can mark on to measure your wrist and lay the string back out on a ruler to get your measurement in inches.