I get this question a lot, especially during holidays when people are purchasing gifts for loved ones. Here is my suggestion on finding out your (or someone else's) ring size.
1. DO NOT GUESS! To resize a ring is a cumbersome and expensive endeavor.
2. Visit a local jewelry store! They have ring sizers and would be happy to size your finger, whether you buy from them or not.
3. Use this website! I have found this to be a credible method of getting your proper ring size. Carefully follow the directions - Ring Sizing