Note: Plastic frames generally have fixed bridges that must fit the bridge of your nose comfortably, with the pupils of your eyes positioned in the middle to upper half of the lens. Frames with pads allow you more flexibility because they are adjustable. Pads can be adjusted by placing your forefinger and thumb on pad and gently moving it one way or the other to make bridge wider or narrower. Hold the frame by the bridge with the other hand. The bridge fitting differs slightly from person to person, whether it be wide, narrow, high, low etcetera; but most spectacle wearers know when a frame fits comfortably, with their pupils in the upper half of the lens. As for the arm length, for a more comfortable fit, it is better that each arm is no more than 5cm past the top of the ear. Most high street Opticians will fit your frame either free of charge or for a small fee (£5 - £10). In many cases this will not be necessary as modern frames hug the contours of your head rather than adjust down over the back of the ears.