RING SIZING
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. The important thing is to measure the INSIDE of the ring as per in the photo below. The thickness of rings varies so it is the inside diameter we are looking for. 
Once you have the inside diameter refer to the below chart to find your size. 
Inside Diameter  US Sizes  Australian Sizes 
14.86 mm 4 H 1/2
15.27 mm 4 1/2 I 1/2 
15.70 mm 5 J 1/2
16.10 mm 5 1/2 L
16.51 mm 6 M
16.92 mm 6 1/2 N
17.35 mm 7 O
17.75 mm 7 1/2 P
18.19 mm 8 Q
18.53 mm 8 1/2 Q 3/4
18.89 mm 9 R 3/4
19.41 mm  9 1/2 S 3/4 
19.84 mm 10 T 1/2 
20.20 mm 10 1/2 U 1/2
20.68 mm 11 V 1/2 
21.08 mm 11 1/2 W 3/4
21.49 mm  12 Y
BANGLE SIZING
Inside Diameter
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.