The lens diameter is the width of the lens measured from the bridge to the outer edge of the glass.
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.
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).