To select a perfectly fitting frame online is simple, to know your frame size it's easy to find on your current frame
Please see the below image to find the frame size
usually medium size frames will fit to most of the customers.
Sunglasses tend to be larger than eyeglasses, so your sunglasses size will most likely be different than your eyeglasses size. With sunglasses, you have more freedom to choose within a wider range of sizes, since the fit does not need to be as precise.
As these are not standards, but broadly classfied frames sizes as per below
Small : 47 mm to 50 mm
Medium : 51 mm to 53mm
Large : 54 mm and above