Many models of glasses are available in sizes, small size, medium size and large size,
depending on the size of each person's face. This is the case with Emblema glasses, for example. If you don't know how to choose your size, here is a simple guide to locate it.
You can measure your face in a few steps, using a credit card. It's definitely an alternative way, but it works: the height measurement of a credit card is very similar to the width measurement of a standard lens.
Set yourself up in front of the mirror or the front camera of your phone and place the long side of the credit card in the center of your nose, the short side under your eye. And find out your size.