I just made a whopper of a mistake by hanging a 24"x30" mirror vertically over a 48" vanity. It's already been switched to horizontal! (For a price, heh.) I suggest cutting out shapes and sizes and playing with placement when the vanity is in place. Or do it on graph paper if you like all the arithmetic involved.
I just checked our mirror, 30" probably isn't wide enough for the peeps standing off center, but 36" would be. Also seconding the rectangular mirror with rounded corners. Ours looks great. I love the way it looks. As for black or wood, I would probably choose a natural-finish wood for the warmth and variety, BUT ... One of the "rules" when mixing finishes is don't use any finish only once in a space. Is there another place to use wood? Maybe floating shelves over the toilet? Or a wooden box on the top of the toilet tank? All the best! This is a fun thing to do, but really, all these decisions can be stressful. I am finally down to choosing artwork. Yay!
Q