A mirror that is slightly the wrong size is more noticeable than no mirror at all. The good news is that almost all of it comes down to three numbers.
Width: two thirds of what is underneath
Hanging above a console, sideboard, sofa or bed, aim for a mirror roughly two thirds the width of the furniture below it. On a 60 inch console, that is a mirror around 40 inches wide. Wider than the furniture looks top heavy. Much narrower looks like an afterthought.
Over a bathroom vanity, run the opposite way: match the vanity width or come in one to two inches on each side. Never wider than the counter.
Height: centre it at eye level
The centre of the mirror should sit about 57 to 60 inches from the floor. That is standard gallery hanging height and it is what your eye expects, whether or not you know why.
If it is going above furniture, there is a second rule that usually agrees with the first: leave four to eight inches between the top of the furniture and the bottom of the mirror. Less and it looks crowded. More and the two pieces stop reading as a pair.
Full length: the minimum that actually works
To see your whole outfit you need at least 12 inches of width and 48 inches of height. That is the floor, not the target. A 65 to 70 inch tall mirror gives you a clean head to toe view without backing into the opposite wall, and it makes the ceiling look higher as a bonus.
Where to put it
A mirror is a light tool before it is a decor piece. Hang it opposite or perpendicular to a window and it will bounce daylight deeper into the room. Hang it facing a blank wall and it reflects a blank wall.
Two more that are worth knowing:
- In a narrow hallway, a mirror on the long wall doubles the apparent width
- Above a fireplace, keep the mirror inside the width of the mantel, not overhanging it
Before you drill
Cut the mirror's outline out of kraft paper or newspaper and tape it to the wall. Live with it for a day. It costs nothing and it has saved a lot of walls.