Can you do a mockup? Use painters tape in the dimensions of your vanity, and your mirror (unless you have the actual mirror & can hang it on a wall at the correct height) and then either use painters tape or a cutout of a sconce you like to see how it would look. Here's what I did:

The bump out in my bedroom happened to be 60", the same width of my vanity, so I just used that. The painters tape is at vanity height.

Thank you! That is a great idea. I'll give it a try. Now I just need to find a sconce I like! Your bathroom looks lovely, by the way!

Also, do a search on Houzz photos for "vanity sconces" or "mirror sconces" or something like that. Look at the proportions in the photos and see what pleases your eye.

We are re-doing our master bath, with the same shower tiles and also a white vanity with marble top! I think we chose all chrome or brushed chrome fixtures - can't even recall.

Our original plan was also marble hex tiles for the floor. We changed that to white porcelain hex to avoid two different marbles: close, but no cigar. You might want to verify that your two marbles work well together.

In this case, I really don't like the idea of mixing the metal finishes. Pick one finish. You could have mixed black with chrome or black with brass. But chrome faucets with brass hardware and lights looks like a mistake.

@stiley Thanks for the suggestions! That middle sconce from Rejuvenation is actually at the top of my list, except that I think I'd hang it upside down...and I was just checking out that same sconce on etsy! I think we have the same taste...I'm wondering if the sconce is the place where I can get a little funky...I think of the sconces like jewelry! But, at the same time, I'm also wondering if I'll regret it if I go crazy...

@BeverlyFLADeziner Thanks for all of these suggestions. they are all great. I'll have to give the idea of mixing metals more thought...I've also thought about going with all chrome and doing a wood-trimmed mirror and some wood shelves over the toilet instead to warm up the space instead of the gold sconces....

@latifolia Thanks for the tip! I'll have to check it out. I was going to go with a more expensive vanity with a higher-end marble top but decided to go cheaper so I could splurge other places, so the marble isn't as nice as some I'd seen. I brought my marble hex sample to the showroom and tried it there and it seemed okay, but the lighting was really weird...and I am excited we'll have twin bathrooms! Is yours done? I'd love to see the blue tile on the walls if you wouldn't mind sharing...

I like this diagram from Circa Lighting. (I've been spending way too much time on their website lately.) I think once you get a feel for how high to hang them you can then look at the scale in relationship to the height of your mirror. I wouldn't go skimpy - 1/3 to 1/2 the height of your mirror at least. When you look at picks of high end bathrooms, they tend to have larger sconces that take up space and are a fantastic decorative feature.