The ring should fit you comfortably; it should be tight enough not to fall off but loose enough to slide over your knuckle.
Measure the circumference of the finger where you are going to put the ring with a sewing meter, if you do not have a sewing meter you can cut a strip of paper, surround the finger with the strip and make a mark.
Then measure that section of the paper with a ruler or tape measure, the measurement you get is the circumference.
Measure the finger a couple of times to avoid mistakes.
circumference: XS(50mm), S(54.3mm), M(58mm), L(61.8mm), XL(66mm)
See more detail:
Size Guide
