There's an ad below where I'm typing the says you can receive their "free ring sizer to ensure you choose the perfect ring" at www.bluenile.com. There are also many places where you can download a ring size chart to print out and compare the length of a string you've wound once around your finger and slipped over the largest knuckle. Remember, too, that a ring with a wide band will need to be larger than one with a narrow band because it's harder to slip a wide band over a knuckle. I've done this for rings I've bought online. But I've also found that there may be slight differences in sizes between sellers, so be prepared and make sure you check out the seller's return/replacement policy before buying.
They have ring size charts... they are pieces of paper with holes to stick your finger through and they get larger and larger. Which ever one fits you that is the size that you are. It says the size underneath each perfectly circular hole. I have one here at home.