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

These eyewear size numbers are always in millimeters so we can get a more accurate measurement, since we are measuring a very small area like on an eyeglass frame.

The eyeglass numbers are usually written as:  49-19-135 or they might have small boxes between the numbers on the eyeglass frame.
1. The first number in the eyeglass size is the distance across one lens of the eyeglass frame ( does not include the frame)

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

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

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