Distance is irrelevant, it's magnification that matters for power/brightness, i.e. image size. For the side of a building and decent brightness, you need a thousand watts or three. You get the same effect from a short lens up close as a long lens further away as long as their relative aperture is the same.
As choosing the focal length, it's a function of your slide size and how far away you want the projector. Same geometry as with a camera.
There are occasional events where people project patterns on whole buildings and that particular one is done 5kW lamps shining through 5" mylar rolls that advance every two minutes; I think the image is around the 5x7" size. They're 4-story-high images though and projected onto dull sandstone - if your building is white or concrete, you will get better brightness.
I would suggest you look at getting something at least as grunty as a Goetschmann G67 (it's like a Slide King but more modern; takes the 85mm mounts). 35mm WILL NOT cut it because the optical flux through the slide required to light up a building will melt it. If you wanted to hack a Slide King or similar for wide angle, you could start by mounting shorter RB or RZ lenses to the front.
Of course you can get a digital projector to do it, but you're talking about a cinema-class Barco for many tens of $k, not a powerpoint-thrower designed for office use.
Last edited by polyglot; 01-10-2013 at 05:59 PM. Click to view previous post history.