The best guide for you is to look at your current spectacles. The measurements in millimeters can usually be found on the inside of the frame, on the arm or on the nose bridge. They will look something like this: (example) - 46-20-135
If the measurements are worn away then a mm ruler can be used to measure your old glasses.
1st number - Lens width: Related to your eye size, and typically in the range of 46 (small) to 56 (large) (where the lens is)
2nd number - Bridge width: This is where you nose sits. Typically this is between 15 and 20.
3rd number - Side Length: 130 (short), 135 (medium) and 140 (long) (from lens to the end piece of your ear) We don't quote this number as a high percentage of frames are size 135
These sizes are a guide to comfortable fitting. Simply select one of our frames that is nearest to your size when ordering.