The size of the mirror doesn't really matter, so long as you can see the door. You could get by on a very small mirror indeed. Regarding shape - as it will be in the NW, you could use a round mirror (circular shapes are metal), so it will double up as an activation for your heavenly and mentor luck in the general space Feng Shui sense. A square (earth) mirror is also possible, as earth produces metal.
Kaye
