The basic rule of thumb is too use a vanity light that is no wider than the mirror. You don't want to go too small either, so maybe something just a little smaller than the width of the mirror.

Hope that helps.

if you have space on each side of the mirror - it is best to have lights placed left and right for the most coverage on your face and neck. The sconces need to have quality glass so you do not have harsh glare from the light source. If you need to have a light placed above the mirror instead due to wall space restrictions- make sure you have a fixture that will distribute light in all directions, such as from frosted glass, so light will reflect around the room for lighting angles of your face as best as possible.

Its for my 13 y/o's bathroom, so I'm sure he not concerned about make up issue. The room itself has a light as well. I really just need to figure the measurement so it looks appropriate.