I think those sconces are too small and won't provide much light. I hope you have several recessed lights too.

When going with higher ceilings you need to scale up your other fixtures so that things look proportional.

I have 10 ft ceilings in my bathroom. My mirrors are 48" high and my sconces are 24" high. The width of my mirror is based on having 2 sinks and 3 sconces above one vanity, so it doesn't relate to your space.

There isn't one correct answer for what the width of your mirrors should be.

You have a scaled drawing, so use that to draw out a couple of mirror sizes using the same scale and place them on the drawing. Scale out the sconces too and add them to the drawing. This should give an idea of what looks "right" to your eye.

If the vanities are in place, then buy some poster board and make mirror mock ups in various sizes and see what looks best in the actual space.

I'd choose the same style mirrors in different sizes based on the vanities. I used butcher paper to see the scale when I did my last bathroom and it really helped get the right size. Love your choices! Can I ask the make of your vanities, I'm looking for that style and having a hard time the right one in the size I need.

I think you should consider working with a lighting expert. I'm doing it with my build and she came up with solutions that I would've never come up with.