If you already own a pair of glasses and they fit you well, you could check the ones you have to see a rough idea of the size you are needing or if you saw something in a store you liked, just write down the size if it fit you well.
Usually, the size is written in 3 consecutive numbers, the first 2 numbers are 2 digits and the last a 3 digit number, this is usually on the inside of the temples, like this 58-16-145
All measurements are in millimeters. It only has the numbers on the frames, but it'll be like this 58mm-16mm-145mm
The first number is always the eye size (width of one lens) so the lens size would be 58mm
The second is the bridge (nose part) width, so the bridge would be 16mm
The third is the temple length, this is the total length if it were straighten out, as all glasses are bent near the end to wrap around your ear. So the temple length is 145mm