1 - Take a credit or debit card
This is the approximate width of a Standard Size lens.
2 - Stand in front of a mirror
Position the side of the card on the center of your nose and ask: where does the other side of the card end?
3 - Discover the size of your lenses
A. If the edge of the credit card ends at the end of the eye, Standard Size is the correct fit for you
B. If it extends well beyond the end of the eye, you need the Small Size
C. If, instead, the credit card does not reach the end of the eye, you should go for a Large Size.