Generate jewelry ring sizer with whatever sizes you want.
You can create:
1) Individual ring(s)
2) A solid one-piece tool with several rings
3) A one-piece tool in two pieces so people don't get their fingers stuck. (Pictured. Notice how you print TWO of the generated stl and tie them together with twistie ties or rubber bands.)
For convenience, I've added STL's for odd sizes 3-11 and even sizes 4-12.
This is based on the maths of http://www.thingiverse.com/thing:7210 and the sizes appear to be correct according to three people I tried it on. Maybe a tiny, tiny bit loose if anything, but seems to be really close!