What is the best SIZE for scanned images that will be projected using powerpoint or keynote?

Typical computer monitors’ resolution is only 1024×768. Any more than this is really just wasted when you’re looking at it on the screen.

When I worked with Adobe Photoshop, I’d usually make my images screen resolution or smaller (for electronic presentations—much higher for prints). It works out to about 650KB.

Thanks. That helps. Is the best way to achieve that resolution (1024×768) to scan the image into Photoshop at a lower resolution, or should I still scan at 1100 and then change the pixel dimensions or the document size?

I would scan it at the highest resolution you can and then change the size in Photoshop. That way, you’ll get as much detail from the original as possible. Plus, if you save a copy at the higher resolution, you’ll have both in case you ever need it. You can always decrease resolution without a loss in quality, but never increase.