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.
Once you have the 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.