How big should text be? What's the ideal font size?
How can you be certain that everyone in the audience can read the text on your slides?
While the fonts and colors you choose have a definite impact on legibility, the single biggest factor is text size. The 8H rule is the time-honored way to make sure that even the folks in the back row can read the text on your slides.
The 8H rule says that the maximum viewing distance shouldn't be more than 8 times the height (H) of the screen; if that condition is met then as long as your text is at least 1/50th the height of the screen, then it'll be legible at the maximum viewing distance. That assumes that the person in the back row has good eyes, that the projected image is perfectly crisp, and that no other factors interfere. And it's an absolute minimum, not a recommended size.
A normal screen show slide in PowerPoint is 7.5 inches or 540 points tall, so the absolute, don't go below it minimum text size would be 540 / 50 or roughly 11 point text. For 35mm slides or good quality overheads, that's not unreasonable. For screenshow projection, it's wildly optimistic. You simply can't form legible text at this height ... roughly 12 pixels ... in most fonts. For projected 800x600 screens, I'd at least double that, or use 1/25th the screen height to determine minimum text size. That translates to roughly 22 points. Use 24 points to give yourself some extra leeway in case projection conditions or your audience's vision aren't perfect (they won't be).
Keep in mind that your screen may not be big enough to meet the 8H rule. If not, you'll have to compensate. For example, if the screen is 5 feet high in an 80 foot deep room, it's only half the recommended size, so you'll need to double the minimum text height to compensate.