If you own a pair of glasses, you can easily compare sizes by finding the size of the ones you own. Inside the temple (arm) of your glasses, you'll find three numbers to help identify the size.
|Frame Size||Eyeglasses Lens Width||Sunglasses Lens Width|
|Small||Below 48mm||Below 55mm|
|Medium||48mm to 55mm||56mm to 64mm|
|Large||Above 55mm||Above 64mm|