Bangle Sizing - It is the size of the widest part of your hand, rather than your wrist that needs to be measured, so that the bangle can be slipped on and off. 

Measuring your hand - First make a fist - then measure across your knuckles, from the outside of your first knuckle to mid way between your third and fourth knuckle.  This distance should equal the internal diameter of your bangle.  eg if this measurement equals 6.5cm, then your bangle will need to be 6.5cm

Alternatively, measure a bangle you already know is a good size for you.  Measure across from one side to the other to obtain the internal diameter.