Jeannie, it depends on both the look you want as well as the light you need. There's no rigid rule for the "right" side light bar, but probably 30" to 36" centered over the 48" vanity would look good. But it may also depend on the fixture you pick: how many lights does it have, how much light do they put out, and what kind of material is the light shade?
For example, you could have a 36" fixture with three 40 watt lights and heavily frosted glass that would put out much less light than a smaller 24" fixture with three 65 watt halogen lights and clear or slightly frosted glass.
You also need to see if the lights direct much of the light up, down or out. For example, a fixture with its lights pointed up could be fine for providing soft illumination bouncing off the ceiling to light the room, but not provide much task light for your activities at the vanity; a fixture with its lights directed down could provide excellent lighting for the vanity, but leave the overall room feeling a little dark.
It's best if you can see fixtures in a couple lighting showrooms, because these factors are often hard to judge looking at pictures in a catalog or web site. Even if a showroom doesn't have the exact fixture you want, if you see others by the same manufacturer it can help you gauge what you feel will work best for you.
-- Eric
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