size against the res of the projector. if it's a 1080p projector, size to that. if it's a 1024x768 or 480, size accordingly. you won't see any real benefit over-res'ing your images. maybe up your dpi a bit, but not to the point where the image sizes choke your presentation computer.
Most projectors are absolute shite, unless they've spent a lot of money on a good one. I've found that even good universities are usually mediocre projectors. Plus the viewing distance will be quite far.
I'd say go for images which the computer (presumably a laptop) has no problems displaying.
The other thing is to make sure you adjust the brightness and contrast of your images. Projectors are often quite flat, and don't display black or bright white very well.