Or you can look on your current pair of glasses and they have numbers on them that tell you different measurements. On the bridge should be two numbers which is your lens width and your bridge length. On the left arm there should be the same two numbers as on the bridge plus a 3 digit number close to the left lens. The third number is your arm length. Get as close as that to possible.
You might have to take them in to have them shaped around your ears, but at least you're not spending $300 or $400 to get a pair of glasses.