Since nobody responded, I thought I would add my 2 - well, make that 1 - cent. I have worked with quite a few projectors, but certainly not as an art photographer. On the whole, the issue is quite messy. I am using a Canon XEED SX7 projector in house (and a bad Philips as well). Different projectors can yield _very_ different results.
Size: I would go for native resolution for the image. Downsample them in Photoshop (or any other decent resizer) to the exact size you are going to use. There is the potential for a lot of conversions in the output chain - OS, presentation program, projector zoom, etc). If you go "pixel for pixel", you can't go wrong. Colour space: that particular projector supports sRGB space - I believe most decent projectors do nowadays - but it may have to be set explicitly in the menu. Note that in addition to colour space, most projectors offer "modes" - presentation/photo/movie that have different colour balances. Last but not least, they also have different power levels for their lamps (often "silent" and "normal") which behave quite differently. Obviously, the lighting (or lack of it) of the presentation room matters a lot. For me, in a very dark room where I have total control, I use sRGB, silent mode, movie colour balance to get a warm atmosphere. Sharpening: unless the presentation is done on a perfectly straight screen, with a very careful focus, your image will be a bit less than perfect in terms of sharpness. I would not hesitate sharpening a bit more than what I would do for full resolution on a large LCD. But there is no magical recipe imho. Come early, and experiment a bit with the projector's settings. The ones I've used offer much more control over the image parameters than a typical monitor. Occasionally, you'll stumble upon low end (or old high-end) projectors that can't be coerced into anything decent regardless of what you do.
If you have plenty of time, and a good projector, you can of course calibrate it with a colorimeter.
My thanks too, Pierre. I've borrowed a Sanyo PLC-WXU10B which gave terrible colour until I found how to set it to Adobe RGB, which is the colour space of the JPEGs I was projecting. It's certainly not perfect, but is acceptable for a non-specialist audience. Do you think sRGB JPEGs might work better?