If you already own a pair of glasses that you find comfortable, we recommend that you take the measurements from these.
A frame has three basic sizes: 1) eye size, 2) bridge size, 3) arm size (measured in millimetres and always written in this order). These sizes will often be written somewhere on the frame, usually on the arm or behind the bridge in a sequence of numbers, such as: 48 18 140.
See the diagram below:
Sometimes the numbers will have worn away and you will need to measure them yourself.
We recommend that when you order your new frame, the size is within +/- 4mm of your existing frames.