Here are the reference that for the size of the bangle.
Put you hand in the same position that when you slip the bangle in.

Then measure the widest part around you hand (Circumference)

Your hand measure outcome:  (allow you to wear the bangle on and off)         
under 6 3/4 inches  (2 1/8 inches inner diameter bangle) Extra Small 
under 7 1/4 inches  (2 1/4 inches inner diameter bangle) Small 

under 7 3/4 inches  (2 3/8 inches inner diameter bangle) Medium
under 8 1/4 inches  (2 1/2 inches inner diameter bangle) Large
under 8 3/4 inches  (2 5/8 inches inner diameter bangle) Extra Large

If you wear the bangle on for permanent. deduct 1/4 inches from the bangle's diameter.
To accomplish this, you need to apply lotion or soap on your hand when slipping the bangle on.