That looks like the convention of about 4" in the picture...maybe you could go 6" to reduce splashing--or you could go all the way to the counter if you don't mind cleaning the bottom of the mirror. I probably would not ease the top of the splash, I think I would want a straight edge-to-straight edge relationship between the marble and the mirror.

I personally really like the look of boundary to boundary mirror like this (whatever the boundary may be). That is a great looking set up in your sample picture. One of my favorite small powderrooms has a counter to ceiling wall to wall mirror with a chandelier hanging in front of it. I think its clean and timeless, not d---- like some people like to call it:)

My RH vanity has a 3" high marble backsplash with an edge that mirrors the countertop. (I am not quite certain of the name of the edge. It's quite flat/square with just barely rounded corners.) I would agree with palimpsest that the inspiration picture is probably 4".