These are great questions!
Drawn from our guide to light sizing: http://bit.ly/1cuh1Fj

Allow 4 inches of space on either side of the mirror that you want to flank with sconces. That 4 inches provides the space for the sconce's electrical panel.

Mount sconces 36 to 40" apart, flanking the mirror, 18" from the sink's center line. If the sconces have shades, put the bottom edges of the shades a little below eye level (60 to 68" from the floor).

I hope this helps and let me know if you have more questions!

@Lights Online, I stumbled across this discussion while trying to find the answer to the same question as the OP. So my builder is putting a 48" single sink vanity in the master bathroom which has a wall on one end of it. I'm getting a 40" medicine cabinet above the vanity which will leave only 4" on each side for wall sconces that my wife really wants. Is that really enough space for wall sconces?

Sam K, I have the same situation. How did it work out for you?