The easiest way to get the right frame size is to get the dimensions from a comfortable pair of glasses you already have.
On one of the legs you will usually find (with the aid of a magnifying glass) three measurements, the first two separated by a rectangle.
The Lens Diameter is a matter of choice, but the other two determine the fit.
They do not need to be exact, but the closer they are to your existing pair, the more likely your new glasses are to be a comfortable fit.
Image showing how to read your frame dimensions