Just google "ring size chart" and you'll find multiple examples of ring sizes converted to mm. There are different charts for men and women, but as an example, a size 6 is typically 16.5mm. Another common ring size is 9, which converts to 18.9mm. Just be sure you have Blender set up correctly to model in millimeters and you should have no problem with this size conversion.
Be sure you are using the measurement tools available in Blender. Jewelers will use what is known as a mandrel to gauge the ring size to fit. In Blender, you can create your own "mandrel" using a cylinder. Just create a smooth cylinder to exactly the scale you need the ring in millimeters, then create your ring around that cylinder to ensure it is gauged properly. Then always measure the ring afterwards too using the measuring tools in Blender.
