Many web sites that sell glasses have methods the determine the best frame size. But this is very hard to do without actually trying the frame on yourself. This is why Yankee Optical is perfectly happy to look at your picture and recommend some frames for you.
If you have an old pair of glasses, you may actually be able to determine your frame size because it many cases, it is written on one of the stems (also called temple) of your glasses as illustrated below.
These numbers refer to the dimensions of your glasses and are measured in millimeters (about 25 millimeters equals one inch). In this case, the 55 mm is the width of one lens including the frame. The 16 mm is the width of the bridge of the glasses that connects the two lenses. The 140 mm refers to the length of the stem (or temple), the part that goes from you frame to your ear. Perhaps a picture will make this clearer.
With these numbers, you can determine what frame size to pick based on your face as seen in the chart below:
The same applies to temple size. Children or smaller faces have temple sizes 135 mm or less. A medium sized face is 136 mm – 140 mm. A larger face is over 140 mm.