The most common bangle size is the Medium one. It has a 65mm internal diameter.
Only size down if you have very dainty hands or up if your hands are larger.
To check your sizing start with the
Medium:
Take a thin piece of card and cut a 204mm length. Tape the ends together and slip over your hand.
Large:
Take a thin piece of card and cut a 220mm length. Tape the ends together and slip over your hand.
Small:
Take a thin piece of card and cut a 189mm length. Tape the ends together and slip over your hand.
It is important to be accurate when measuring the mm.