There are two methods to measure its ring size: 

Either you can measure the inner diameter of a ring that fits on the finger and times the number of mm by 3.14159263359 (= pi). The result is equal to the ring size.

Another option is to measure the relevant finger with a piece of tape that is both inelastic and flat as, for example, a gift band. Put the tape around your finger and make sure it is neither too loose nor tight. Select the start and end of the ribbon and you'll have the ring size.

Most rings can be resized in a jeweler, so if you have found the ring, but the wrong size, it may still fit.

Remember that the thickness of the fingers changes with the temperature outside.