I would leave enough room so that when I turned on the faucet I was not at risk of splashing my hands onto the mirror. If the mirror is on a medicine cabinet, be sure the mirror, when opened, clears the faucet handle when "on" If neither one of these issues are pertinent, then I would be sure that there is room above the counter so the bottom edge is not exposed to water. We had a mounted wall to wall mirror above the vanity and it rested on the 4" backsplash. Over a few years the mirror delaminated near the faucet/sink wet area. The mirror would get wet and run down into the bottom space between the mirror and the top of the backsplash. It just got worse and worse over the years and because it was affixed to the wall, there wasn't anything to do but tolerate it until time to do a gut of the room.

As far as width, that would be your personal preference. If the mirror is on a cabinet, will the door swing interfere with side lights?

We have 27" cabinets for our sinks and actually went with a 28" mirror so it really goes across the whole space. For backsplash height, I also took into account the faucet height. The border going there was our most expensive tile and I wanted it to be seen! I can go take exact measurements but our backsplash is about 6" and then there is a 2" gap up to the mirror.

Don't forget to account for any pencil on top - I did that in my girls' bathroom remodel. We measured out how many glass tiles I wanted but forgot that there would be a pencil on top. :)

anyway, in our master, I find that I can't really splash the mirror which I really appreciate b/c it's decorative.

Pics below - good luck!

Alexa

Master:

Close up on mirror from backsplash:

Girls' bathroom:

This post was edited by akcorcoran on Wed, Aug 21, 13 at 16:00