The notation for the size of optical frames is universally standardised.

This size of a frame is denoted by a series of three numbers, usually found stamped on the inside of one of the arms, with the first 2 measurements usually (but not always) separated by a square symbol.

As an example, the particular frame shown below has measurements of:

51 □12 - 132

The first number is the diameter of the lens:  51 mm

The second number is the width of the bridge across the nose:  12mm

The third number is the length of the arm:  132mm