I found 34" to the top of the vessel sink to be what several clients have decided on when installed. Subtract the sink height from 34 , and you will get your counter height. That being said, a sink height anywhere's from 32" ( low) to 36" high ( ok if your tall) is cool.

While I don't know the width and height recommendations for a double vanity, I do know that in such a situation, it can look really good to have a total of 3 sconces: 1 flanking the left side of vanity A, 1 between vanity A and B, 1 flanking the right side of vanity B That way, both vanity A and vanity B have two lights flanking. [Here's[(https://www.houzz.com/photos/bathrooms-traditional-bathroom-boston-phvw-vp~579223) an example of this in action. If the two mirrors are not very close to one another, as in the case of [this bathroom for example[(https://www.houzz.com/photos/traditional-bathroom-bath-vanity-traditional-bathroom-denver-phvw-vp~572721), then you can have a total of 4 sconces.

I would keep them eye level as long as you use incandescent bulbs. That style bulb emits light in a 360 degree spread and will hit the middle area. If you use LED or fluorescent screw in, the light spread is different and you would have to adjust your height for that. LED being the least round illumination.

It depends the light you choose. You need light that points down. That assumes they will have a glass shade, in most instances. Which means you will be better off with a 42 inch mirror and that is plenty tall enough. It also means you will need TWO HOOKS on the back of the mirror, one on each side and forget wire. You won't have room to lift up and ONTO a hook using wire, even with the shades off in most cases. Two hooks on the back will ensure they hang straight after cleaning the spots off too : ) Must use these . In any casem hard to help without knowing WHICH light and its specs.