Each frame has a series of 3 numbers imprinted either on the inside of the temple, inside the bridge(across the nose piece) or in a plastic frame it is sometimes stamped on the nose piece.

These numbers are always in millimeters so we can get a more accurate measurement, since we are measuring a very small area.

The numbers are usually written as:  49-19-135
1. The first number is the distance across one lens ( does not include the frame)

2.The second number is the distance across the bridge or sometimes refered to as the DBL(distance between lenses)

3.The third number is the length of the temple(arm piece) from the front of the frame to the end of the temple, which goes behind the ear.

If you are looking for a frame on Ebay and you already wear glasses, I would suggest you check the numbers in your exsisting frame and and find a frame with similar sizes on Ebay.  The other thing you can do is go to an optical store, try on some frames and copy down the size of the frame that is most comfortable. This would be the set of 3 numbers we talked about above. Use this as a guide for finding a frame on Ebay.