It's really impossible to say, Chelsea.

You really want to think in pixels. Think of it this way -- if your display
is set to 1024 x 768, then when you run your slide show, the slide takes up
1024 x 768 pixels. So if you want your picture to fill your slide, make the
picture 1024 x 768. If you want the picture to fill half the slide, make it
(about) 500 x 380.

Now to translate that to kb.

I opened Photoshop and created a file that was 1024 x 768 pixels. I filled
it with white. That's all it is. One big expanse of white.

PNG = 4 kb
JPG (12 compression) = 71 kb
JPG (6 compression) = 33 kb

Next I opened a photo and cropped it to 1024x768 pixels.

PNG = 1456 kb
JPG (12 compression) 1136 kb
JPG (6 compression) 162 kb

As you can see, there is a huge amount of variation in the kb sizes for each
image -- even when you save as the same file type. (And then there's
grayscale vs RGB and all kinds of other stuff.) So just think in pixels and
you'll be okay.