If you've ever needed to know your ring size, print away, or if you are buying a custom ring from overseas, why not mail a ring that is your exact size across to them?

Check out an updated version of the rings here:

http://www.thingiverse.com/thing:439229

Print up a few if you roughly know your size, print up the lot if you need to 'catch em all'.
All rings are 4mm thick, depending on the type of ring you are getting made, you should scale the ring in the Z direction to approximate the thickness of the intended band. (Fat Ring = Fat Ring sizer, Skinny ring = Skinny ring sizer).
I needed to find out my girlfriend's ring size to get an engagement ring made, (shh! she still doesn't know).
If you want to convert these British / Australian sizes into your local measurement, try this site: http://www.onlineconversion.com/ring_size.htm (It is where I got the sizing from).
*Be carful, your 3D printer may not print a perfect size ring! That's why I posted my selected ring to the jeweller.
I figured I'd make the rest of the sizing rings, as someone will need these one day.