I recommend going to a jeweler and having a measurement done of the finger you intend to wear the ring on. You can also buy a ring sizer like this to get your measurement.
IMPORTANT SIZING INFORMATION: These stone rings can fit differently than standard metal rings, I always recommend going at least a half size higher than what you normally wear. Remember: 1) the piece is hand carved, so it may not be a perfect circle, 2) stone doesn’t expand and contract like metal and your finger does, so if you get hot or drink a bunch of water, it may become hard to remove, 3) the bands tend to be wider than metal rings and so harder to get on and off. If the size ring you wear is tight on your finger, go a full size higher, so you can ensure it will come on and off with ease.