You're in there too short of a time to get tired or overwhelmed by them. So that's a perfect place for a little drama. The usual rule, that dark colors make a small room seem smaller, although true, can be overruled here. I would typically not use a pattern, not even a small one, in a small bathroom. Next, the mirror.
Many rooms have a flat mirror attached to the wall.
In a powder room, I like a beautifully framed mirror. I love ovals, but rectangular mirrors work well over sinks, too. For lighting, remember, the light level can be low in a powder room. Don't use an overhead light that shines directly on a person's head because it casts a ghoulish shadow on their face. I like wall sconces on both sides of the mirror.