Surprisingly, eBay has a good howto guide on how to size rings. (Here’s another PDF version)
Know what size ring you need before you shop so you can buy perfectly-fitting jewelry every time. To determine ring size, you can place a ring that fits you on a professional ring sizer mandrel, use a professional ring sizer, or find an approximate fit with a ring size chart or finger size strip. Make sure to measure your finger at room temperature. Heat and cold can cause your fingers to swell or constrict.