If you have a ring that is the size you are after the easiest thing to do is measure the ring against a ruler to get the inside diameter. Once you have the diameter refer to the below chart to find your size.
Bangles are measured by the internal diameter of the bangle. If you have a bangle at home that fits correctly then simply measure the inside diameter with a ruler. Otherwise to get a rough idea you can lay your wrist on a ruler and this will give you an idea of which diameter should work best.
My standard sizes are:
Extra Small 5.5cm diameter
Small: 6cm diameter
Medium: 6.5cm diameter
Large: 7cm diameter
Extra Large: 7.5cm diameter
As all of my rings are hand made there will always be a slight variation in sizing due to each individual design. I do endeavour to ensure that my sizing is as accurate as possible and I am always happy to make custom sizes that aren't listed as standard options.