Projector's light reflecting in a mirror

Hello all.
I'm trying to play with reflections but I'm having some troubles.
My goal is:
Reflect a light from a DLP projector in a front surface mirror placed with 45 angle in a base, and then, have the exact aspect ratio of the light in a "wall".
Actually, I'm building an open source DLP 3D printer. So, following in attachment, we have some pictures to illustrate the work.
The trick here is. The DLP projector, the mirror and the build platform (wall) must be free to move. The machine will be modular.
Here we have some references:
-The projector has an aspect ratio of 1920x1080 pixels, so my default aspect ratio of the build platform will be 192x108 mm. But the goal is to parametrically increase and decrease this value.
-The mirror if a first surface mirror, so we don't have to deal with mirror refractions.
-When the projector is in the floor, the light projects in a certain direction. (images in attachment).
-The build platform must be right above of the mirror, so this way we avoid distortions.
-The mirror size must be increased and decreased with those light proportions, so this way we can have the exact size of the mirror.

So, who ll take the challenge?
hehe

Thank you in advance