Is the vanity in an alcove? If so, mirror the entire alcove and have the glass company cut the mirror for mounted sconces on the mirror. You could also do this if there's no adjacent walls or one wall, and you may have to. You have 38" to fit a mirror and two sconces (countertop overhangs an inch or so). I would want at least 3" between the sconce shade and mirror and the sconce and the wall, so 3" + 3" + width of sconce. I have a 24" mirror over a 36" vanity, which is a fine size for the mirror, but that leaves only 7" for a sconce between the mirror and wall. If you used a smaller than 24" mirror, the sconces might be too close together. I read that 30" - 36" is the optimal spacing for sconces. I always draw this stuff up on graph paper to check spacing and proportions.

it depends on how close the sink is to the end. I'd scale them as for individual vanities, like 30H by 24W, but you could go a little wider if space allows. Just keep each centered over its sink.

Looks like your sconces might be positioned a bit high

Many people choose vanity lights that are about 75 percent the width of the mirror and center the light above the mirror. You can also choose to use a bath bar light that is exactly as wide as the mirror. An LED mirror is another choice. https://www.lightsonline.com/learn/lighting-ideas-by-room/bathroom-light-fixtures-guide