Is it a center sink base with side drawers (vs. an all-drawer vanity)? What is the width of the sink base? What is the width of the sconces?

I'm going to guess that it's a 24" sink base and the drawers are about 9" wide. I like the mirror to be at least as wide as the sink base. The recommended distance between sconces is about 30-36"--I don't remember the exact numbers but that info is online. I would try to center the sconces over the drawers. That may give you about 26-28" for the mirror width.

This can all be figured out on graph paper to get the correct spacing of the mirror, sconces, and any side walls.

I just drew this up on graph paper and edited my post to knock a few inches off the mirror width. Height doesn't matter, so whatever you're able to find. Uttermost has a lot of nice mirrors.

We don't know the exact measurements of the sink base. If it's 24", which is a standard size, it looks like there's 1" in the vertical trim, a 9" drawer, and 2" in the leg, giving you 12" on each side of the sink base. If that's the case, you could place your sconce 3" in from the edge of the vanity, which you'd really want if there's a wall on one side. Then you'd have 5" for the sconce and 4" between the sconce and the mirror--with a 24" mirror. I personally feel 24" is too narrow for a 48" vanity. I have one over my 36" vanity.

If you're using a bath bar type sconce, they look fine placed closer to the mirror or closer to a wall. You could add those inches to the mirror.

Here's a few photos from my Ideabook. You can see that the mirror is wider than the sink base, pushing the sconces out to the ends, which is okay when there's no side wall.

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If it doesn't work out, I'd consider mounting the sconces on a full-width mirror (46") and then you can space them within the recommended width for best lighting for the face.