I've got a 4 light 30"L fixture over a 36"L mirror over a 49"L vanity in my guest BR - it looks in scale (it's a tall vanity and a correspondingly big mirror), not overpowering in the room, and with CFL bulbs provides a lot of light. I'd suggest just insuring the light is perfectly centered over the mirror.

I am purchasing my bathroom vanity lights this week. For my boy's bathroom, they have 2 vanities back to back from one another. The vanity is 54 inches long. The light fixture I purchased is a 4 light bar, measuring 28 inches across. Is this a good size for that space? In comparison to what we ave in our current home, a 36" vanity with a 24" light fixture. I know it will be enough light just don't want it to look out of proportion. The mirror will extend completely over the vanity and the light will go right above the mirror. I think anything bigger will be overwhelming and too much light.

I took a peek at your ideabooks to see what you like. I think that it would look best with one large expanse of glass across both vanities.

I remember in my first house, I made a recessed closed niche all along the vanity between the faucet and the mirror. I found it really useful, I just wish I could find a better option than sliding doors to access the inside. I don't know, if 30 years later there would be a better option ... maybe you can make that recessed space higher than the faucet and have a pressure button latch that is hidden at the back of the door (sorry, I am missing some english vocabulary, hopefully you understand what I mean).