Shades of Light In this case it maybe more of a wattage issue or personal preference of style since there is only 2" difference in size between the three light (23") and the four light (25"). Ask yourself do I want/need the additional light? More times than not 2 of the three lights are what customers use. However, if the space was unusually dark or perhaps a customer who wants or needs a little more "task" lighting they would go for 2 of the four- light.

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Shades of Light Yes, most definitely. This fixture gives out a lot of light: each arm has a 50watt halogen par bulb that is approximately the equivalent of a 75 watts incandescent bulb with all the light reflected downward. If you are more familiar with lumens each arm is going to give you approximately 1100.
Light comes in a two arm, three arm (as you see) and just recently we added a four arm (because bathroom vanities are getting bigger.)

joyposnick I have a new 37" vanity, how large should my light be above it

Shades of Light More details here http://www.shadesoflight.com/inspired-spaces/tips-and-techniques/bath-light-solutions/
But as a general rule of thumb (and of course there are exceptions once you get down to specifics) I would be looking for bath lights that were 28" or larger

Shades of Light Hello, Thank you. You can find them here http://www.shadesoflight.com/pullman-bath-light-3-light-2-finishes.html Comes in a two Light size as well