The Sunglass Fix and most sunglass manufacturers measure sunglass replacement lens width using:

1) The maximum width

2) As the crow flies, not along the curve of the lens

Manual Measurements using calipers or a ruler:  Allow 1/2 to 2 mm for lens portion under rim.

Using the Sunglass Model Codes to determien Lens Width.

Ray Ban Sunglassses, Persols and a few other brands commonly use the same model number but make several different sizes. For example, the Ray Ban Aviator Large Metal sunglasses, RB3025LM, come in 3 sizes -  55mm, 58mm and 62mm wide.

So how to you determine this?  You can measure them but with two to three millimeters difference it can be hard.

There are labels on the arm or nose bridge that should provide the required information. Additionally, it may be with your original purchase material such as a box or label if kept.