Simon Meeds , Jan 04, 2006; 05:43 p.m.
Ideally you need to know the resolution of the projector and mach the resolution of the computer to it, then assuming you want to show your images full screen you should make your image files the appropriate size: VGA 640x480; SVGA 800x600; XGA 1024x768; etc.
The most common projector resolutions today are SVGA and XGA, though some high-end units probably have higher specification: 1152x864, 1280x720, etc.
If you don't manage to find out what resolution the projector is, then I would suggest sizing your images to XGA. XGA is the lowest resolution which is really useful for photographic purposes and is supported by most modern projectors whether of higher or lower native resolution. The computer should be set up to XGA. If the projector only has native support for SVGA, for example, it will reduce the resolution produced by the computer, if it supports a resolution higher than XGA it will perform interpolation. In either case the output is unlikely to be ideal, but if the auditorium management can't be bothered to let you know the resolution of the projector they can't really expect any better.
Good luck with your presentation.