If you look on the inside of the arm of your glasses you find will something that looks like: 52  18 135.
This is information on the size of your glasses: the lens diameter, the bridge width and the arm length, all measured in millimetres.
THE LENS DIAMETER is the width of the lens (in mm), measured from the bridge. This can be a matter of personal choice, but is also determined by your frame style.
THE BRIDGE WIDTH is the gap above your nose between the lenses. It should be comfortable. Too tight, and the glasses could pinch your nose; too loose and the glasses will not be stable on your face.
THE ARM LENGTH is usually one of three standards – 135, 140 or 145mm. The arm should be long enough to sit comfortably over your ears. Glasses with straight arms do not always have this measurement indicated.