Lens:
The lens diameter is the width of the lens measured from the bridge to the outer edge of the glass.
Bridge:
Bridge width is the gap above your nose between the lenses. Too tight and the glasses may pinch, too loose and the glasses may slip.
Arm:
The length of arm is usually 3 standards, 135mm, 140mm or 145mm and the arm should be long enough to fit over your ears.
All measurements are in millimeters( mm).