Is there a standard resolution for Video Presentations?
If it's NTSC/standard def/4x3, 640x480 will be good. If it's HD/widescreen/16x9, 1920x1080 or 1280x720 (1080p and 720p, respectively). Premiere will likely have presets for all of these aspect ratios. 

I would ask the projector owner for the pixel dimensions or pixel resolution. Likely, it'll match one of the above pixel dimensions. If the owner's not very projector savvy, ask for the model number and manufacturer, and look up the info you need on their website's support section or downloadable manuals.

The thing to keeop in mind is that the projection hardware is a fixed size...usually one of the pixel dimensions noted above, or something CLOSE to it for computer monitor resolution. Even if you feed a 1280x960 image to an NTSC display, it's going to interpolate it into a 640x480 display. On a projector, what makes it bigger is the lens projecting that tiny 640x480 pixel image to a 10' screen. My point is that if you know the maximum pixel resolution on the projector, you don't need to bother rendering anything MORE than that; it won't affect image quality.

good luck!