Wristguard Size Hand Size (Circumference at Knuckles)
JR 6-7 in.
S 7-8 in.
M 8-9 in.
L 9-10 in.

Measure your hand.
To ensure a perfect wrist guard fit, use a string or measuring tape to measure hand circumference at the knuckles and match to the sizes listed above. If your had is between sizes, select the larger size. 

Check for a proper fit.
When trying wristguards on, you shouldn't be able to bend your wrist forward or back. The velcro on the back of the wristguard should line up and shouldn't cut off blood-flow.