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. 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. Otherwise take a piece of string and measure around your finger, remembering that the ring will need to comfortably fit over your knuckle. 
Ring Size Chart
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