The frames come in different sizes. If you look inside one of your frames, you will probably notice two or three numbers.
The first number (usually between 48 and 58) is the width of the lens. The second (normally between 15 and 25) is the width of the bridge, or the distance between the lenses. The higher the number, the wider the distance. The last (normally between 135 and 150) is the length of the temples. If you notice when trying on glasses that the temples don't sit well, you should look for glasses with a temple length of at least 145 mm.