Why not go one step further than hanging a mirror, and create a wall of mirrored tiles instead?
Mirror tiles can make a small bathroom appear larger by reflecting light around the room.
They’re also a decorative alternative to a single mirror above the vanity.
Square tiles can be arranged in square, rectangular or diamond shapes, and are easier to position on the wall above an existing splashback than other-shaped tiles.
Pencil a line down the side edge of one tile when positioned against a base corner.
Use a spirit level to ensure that the line is plumb, then extend the line to the top of the area to be tiled.
Check that the first column of tiles fits neatly in the corner and, if not, move the tile in from the corner rather than cutting it to show where the first column of tiles goes, based on how square the walls are.