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...

There is no rule about the size of light fixtures relevant to the mirror. However, if you choose a sconce, such as in your second choice, you should flank the mirror instead of placing them over it, like you would with the other dual-light fixtures. Any of these choices should be fine. Pick your favorite!

Here is your dilemma. if you take down those fixtures there will be two holes in the mirror, that being said, they will be up high and most likely off center. So, to get new lighting that is not stuck in the corners, and covers the holes behind it, might be next to impossible unless you run a thick bar that covers the space. But some times back in the day they only drug wire over and did not put in actual junction boxes, so the bringing it to code might be another issue. 1. take one of the lights down and see what is going on 2. probably have to replace the mirror and move the electrical location.

Appreciate your reply tartanmeup! I agree about overhead lighting and shadow. - If I were to mount sconces on both side of a mirror and the mounting plate is 4", on top of 4", how much more space away from the wall would this mirror need to be? - I want to have one long mirror, would 2 sconces, 1 on each side be enough for this span? don't really want to go through the mirror for the 3rd sconce in the middle. I don't think mounting through the mirror is in keeping with a modern/minimalism that I'm going for. - If I go with pendants I would try to get them down to my height to avoid shadows. My vanity is 24" deep and maybe mounting this 8-9" away from the mirror down to my height wouldn't get in the way. Would pendants be considered modern/minimalism? I think if I go for pendants, I would use these. I think I have ruled out all the pendants above. At lease this one won't direct the light downward.