A vanity light should hang above mirrors at about 75 to 80 inches from the floor to the ceiling of the light fixture. For mounting beside the mirror, place the vanity light between 60-70 inches from the floor. While this is standard for ceiling and wall mounts, you can always adjust to the height that works best for you.
Vanity lights should be at least 75% of the width of the vanity. Depending on your preferences, your light can be longer than the vanity to add more illumination.