just guessing here - but if the same principles apply to vanities that apply to tables - the width of the light should be roughly half the vanity width? Perhaps a triple sconce for the 48. For a long vanity, I'd probably do two double sconces - so say, 60 divided by 2, so 30, so each sconce roughly 15". Centered over each sink. Of course I'm guessing, and this is without seeing pics. And I don't think it should be much smaller than half, but larger would be OK IMO. (we have a double sconce over a small pedestal sink and it's fine.) But maybe that could give you a starting point...

Are you using side sconces or overhead lights? Is the vanity in an alcove? I would only use a mirror centered over the sink if you're using sconces. Just about anything goes in that case. My favorite look for a full-width mirror over a longer vanity is a narrow one that's raised a few inches above the backsplash. This could be used with either overhead lighting or sconces mounted to the mirror. Your glass company would cut holes for mounting.

Geez Louise, I'm back nearly 3 months later, still pondering this--yikes. Until now, I'd planned to cover most of the vanity wall in a plain wall mirror. But now that my GC covered the entire wall (!) in Porcelanosa Madagascar Blanco tile, I almost hate to cover it up! So now thinking of doing a pared-down single mirror or a pair of mirrors (something I was not keen on previously). But proportion seems a foreign language to me. I cannot figure out what size or shape mirror would look right in this space. Help? FWIW, the floating vanity (which will have clear-coated walnut drawer fronts) is 56" wide and is mounted 36" above floor. The Porcelanosa tile tops out at 7 feet and there's a sloped ceiling above. That giant light switch box is even more annoying in person, yeesh.

In my opinion, the framing around the mirror should match your cabinet, assuming it is wood. As far as fixtures and lighting, I think they should match. We redid our powder room last year and are now revamping our hall/guest bathroom, so I understand you concerns.