I am putting together a slideshow for my son's high school graduation and I've scanned lots of old photos (as JPEGs) into my computer. They are, of course, all different sizes and my question is what size or ratio do I use when I crop/resize them in Photoshop so that they will be uniform and the right size for a slide show?
The right size depends on your intended output for the slideshow. For example, if you intend to burn the slideshow to DVD, you'll likely want to use 720x480.
However, the slideshow editor will automatically resize all of your images to fit the screen, you can then zoom and move each of the files to fit the frame if they are not already the right aspect ratio.