Typically, there will be two or three numbers imprinted on either the bridge (nose area) or one of the temples (arm of the frame) indicating the frames size. All sizes are measured in millimeters.

An example would be either 53-18 or 53-18-142.

a. Lens Width: The first number 54 represents the lens width. It is important to differentiate it from lens height. A large lens for a big guy usually has big sizes at both dimensions. Lens size typically ranges from 44 to 58. 

b. Bridge Distance: The second number 16 represents the bridge distance. It is the minimum horizontal distance between the lenses. It's the size of the 'bridge' of the frame, the part that rests on the nose. This size varies only slightly because an adult's nose will hardly alter in size as they age. The size ranges from 14mm to 24mm.

c. Temple Length: The third number 140 describes the length of the arm or temple. This is the whole length from the screw to its ear tip, consisting of the length of the bend and the length of the drop. The average size for most people is around 140, however some may need either a shorter or a longer temple length.