I have never done this, but you will need a mirror that is equal to the size of the projection. That is, if your projector at say 2 feet give you an image about the size of a laptop screen, then you need a a mirror that size other wise only part of the image will be reflected. If I where you I would see what mirrors you already have and see whether you image is large enough to fill the window after hittig the mirror with the image filling the mirror. If it is too big, move the projector closer till the reflected image is the right size. If it is tool small, then get a bigger mirror.
It really is a case of trial and error as we dont know what projector you have (and that may not help us) the size of your window (and what shape it is) or any other limiting factors.
So when its dark, give it a whirl, have a play inside till you are happy then ho and see what it looks like outside.