Eye size is a term we use in the optical industry to measure frames. If you have a pair of glasses you will be able to see a set on numbers on the inside of the arm. For example, 52 18 130. The first number in the eye size refers to the distance in millimetres across (horizontally) one lens. The second number in the eye size refers to the distance between the two lenses or bridge size (measured in millimetres). The third number is the arm length in millimetres. This is referred to as DBL (Diameter of the lens, Bridge, Length of arm). If you are happy with your glasses, then choose a new pair that is similar in size to your current pair.
If you do not have glasses, then this information can help assist you in sizing a frame for your face. To better understand the frame size, double the first number and add the second. This will give you the distance across the full front of the frame. In the example above, an eye size of 52 18 means that the frame measures about 122 mm across (52 x 2 + 18 = 122). This value should be considered a minimum as there will be some inaccuracies due to the width of any frame and the curve of the hinge. At eyekit.co we provide exact measurements for you which appear on each product page. Armed with this information you can then put the ruler up on your face to see how big the frame will be on you. If you only have inches on your ruler, 25mm equals 1 inch. So, in our example, this frame would measure 4.88 inches (122 divided by 25). You do not need exact measurements; most frames are flexible and comfortable in a range of sizes. You can afford to allow a few mm difference. This will ensure they are neither too tight nor too loose.
Some brands may also give other measurements. The "B" is the height of the lens top to bottom in millimetres. The "ED" is the distance diagonally across the lens in mm. The TPL is the length of the temple or earpiece.