If you look on the inside of your glasses, you'll find a number that looks like: 52  18 135.
This information shows you the size of your glasses; the lens diameter, the bridge width and the side length, all measured in millimetres.
The lens diameter is the width of the lens (in mm), measured from the bridge. It can be a matter of personal choice, but it's also determined by your frame style.
The bridge width is the gap above your nose between the lenses. It's important that this is comfortable - too tight, and the glasses could pinch your nose; too loose and the glasses won't be stable on your face.
The length of the side is usually one of three standards – 135, 140 or 145mm. The side should be long enough to sit comfortably over your ears. Glasses with straight sides don't always have this measurement indicated.