A bangle should be able to fit comfortably over your knuckle, so you don't have to struggle to get it on and off and risk bending it, but you also don't want it to be too lose that it could easily slide off.
So knowing your correct bangle size is important.
If you already have a bangle that fits you well, just measure the diameter and select your size. If you don't have a bangle to do this with, take a strip of paper and make a shape with you hand as if you were going to put a bangle on, measure at the widest, make a mark with a pen, cut off or fold the excess paper, then use a piece of tape to secure the paper to make a circle, then measure the diameter to find the size you need to order.
My bangles come in the following sizes:
Small - Inside Diameter 60mm
Medium - Inside Diameter 65mm
Large - Inside Diameter 70mm
Extra Large - Inside Diameter 75mm
There is also Free UK Delivery.
Have fun and take care