-I think it should repeat the length of the vanity. If the vanity touches walls-then all the wall it is. If the vanity has space between its sides and the walls-then youl leave same exact space

-yes many think so, but sometimes they can be a good solution. For example that's what I did in my master bath-because there's a slight assymetry there created by the window over the vanity..if I'd do two interesting frames and all, as I initially wanted, the assymetry would be much more visible

(I love assymetry btw, but less when it comes to places that already have pairs of things to accomodate..two sinks, two pendants, etc) We did two simple mirrors going to the bottom of the vanity -thus removing neccesity of backsplash, which I didn't want, and making this assymetry of the window almost invisible to whoever doesn't know it's there.

I did another mistake though..didn't take them all the way to the ceiling. (We have pendants, not vanity lights) But. When my pendants break, in some earthquake, maybe then I'll think "good.. now let me just install these vanity lights".

Which brings me to your question of distance-how far above these lights sshould be?. Check the code-electrical box should sit on the stud, and not on the corner one. There's some minimal distance in inches there. The modern box itself is such the fixture should be minimum 5 inches wide to cover it, vanity lights are usually wider. Look for the fixture you like first, read your code/advise with an electrician..or do you have maybe box already there? Then it's all easier. Should be several inches left between the edge of the mirror and the edge of fixture. How many exactly? Enough to be pleasing to the eye. Depends on the fixture, your ceilings' height, etc. Can't tell you exact amount of inches without knowing all the parameters. Even though there might be some basic rules there, like with curtains, rugs,art, etc. I'll try to comb through my saved stuff, if I'll find something I'll post it. Or maybe somebody else will contribute.

Posting photos of your space always help with better advice.

ok..did find something..let me try and attach it

The full coverage mirror is quite popular in some areas where houses do not have gigantic master bathrooms. It's nice to leave space for GFCI electrical outlets on each side. Use the kind with 4 outlets, not 2, so you can plug in nightlight, chargers, and hairdryers without draping the cords over the sink.

I think it mostly depends on what you want. How will the mirror be attached to the wall? With framed mirrors, there is usually space left on each side. Regarding the height, you'll want to keep in mind the height of anyone that will be using the mirror. But they are usually at least 6 inches down from the vanity lights.