These are probably sturdy enough to print and wear for a while just as plastic rings. I was asked to design a printable signet ring to be used for casting metal in Delft clay or sand. Here are ten sizes from 8 through 12.5 (US Sizes). The models shown are printed with a translucent UV resin. 

If you need a size not listed here I can help you out or you can start with size 11.5 which has a 21mm inner diameter. Most ring size diameters differ by about 0.4mm. Find out the diameter you need and do a little math then scale it in your slicer. This formula should work: [desired diameter]/21mm = % to scale to in your slicer. If you look in the notes that accompany the individual stl files I have specified the inside diameter for each.