To find out your ring size, you can:
go to a jewelry store and ask to have your finger measured.
measure the inside diameter of one of your old rings and refer to the table below.
(to be more precise, make sure your ring is round)
measure the circumference of your finger with a string and refer to the table below.
Here is an explanatory video of this technique.
Collars are available in several sizes, 16, 18, 20, 22, 24 and 26 inches. Note that not all sizes are available on certain models.
You can use a string to measure the length that suits you best or refer to the image on the right.