I think it depends on the fixture and mirror. Does the light you picked fit the size and shape of the mirror?

Well I haven't actually bought the mirror yet :) I was planning on a large framed rectangular mirror. This is the light:

Keswick 2-Light Bathroom Vanity Light, Old Bronze · More Info

May-flowers, I was thinking it might look a little puny, it's weird but I like how the wall plate is centered between and the detail of it, which is missing in the 3-light because, well the light is in front of it. :) There is shower light (can't remember bath dimensions, but pretty small, 5x11? Or similar). It's a little dim in there I feel. The 4-light is just a lot more $. I have time so I'm going to shop around. Thank you for sharing that rule of thumb, I have a hard time with visualizations and that is very helpful for me.

I like that light fixture but agree that the "look" of the 3 light is not as balanced and pleasing as the two---

Personally if the light provided by the two light fixture is enough for your use then I would use it...

We are in same predicament--need master bath remodel and will have medicine cabinets that can't be inside wall space so they will project over the sinks/vanity---

I need lights that will come forward enough NOT to have their downward light blocked by top of medicine cabines--those are hard to find but this one I think projects about 7.5" which is likely enough...

loves2read that would be great if this light worked out for you! I really like it. :) I can see how the cabinets would make it difficult.

See now I think three lights look more balanced. It goes along with the design rule of odd numbers being more pleasing to the eye.

Here is a picture of a 2 light over a similar sized vanity. They solved the too little light problem by adding side sconces, but not sure I like that look. Actually the look is fine, I just don't really like the combo they used.

Here is a similar size with a 3 light fixture. I prefer the 2 light

The first example looks good but those are some big lights. The mirror is too small in the second photo in length and width so the light overpowers it. I have a 22" 3-light over my 36" vanity.

DH has the same set up in his bathroom that we're remodeling. I have the 3-light fixture but not the mirror yet. It will be 32" wide. Vanity is 36" but against a wall on one side and we don't want the framed mirror butted up against the wall. I was a bit surprised that the globes were as large as they are. I think the 3 lights will look better. I just hope they aren't unproportionally too large.
http://media.bellacor.com.edgesuite.net/images/500/29515021AZ3.jpg

This is all helpful to get an idea of proper proportion, thank you guys.

If you like the way the wall piece is centered in the two light version why don't you get the 4 light version instead. It has the same look you like with the wall plate centered. The 4 light version is 29 inches wide. If your mirror is as wide as your vanity (or at least 36"wide) then it would extend 3 to 4 inches past the light fixture on each side. It would look fine. If you think it would be too much light you can use lower watt bulbs in it.

I know this is an old thread so hope someone responds. I have a 24 inch reclaimed wood vanity and will be placing a rectangular wood framed mirror above (having made to size based on light fixture used and after marble counter install). I found a light fixture I like and it comes in 2 and 3 light versions (2 is 16 inches wide, 3 is 24 inches wide). Which one should I order? Thanks.

Based on length alone, I'd do two. If the room is large, I'd add overhead lighting too.

I would do double lights over each sink. It seems to be the most common configuration in Houzz photos that I've saved.