Three is the magic number for PowerPoint presentation slides.

3 font sizes:

1. 36pt for titles
2. 24pt for body text
3. 14pt for footnotes

3 font styles:

1. Titles (eg.: Georgia-normal)
2. Body text (eg.: Arial-normal)
3. Emphasis (eg.: Arial-italics)

3 colors for text:

1. Normal text (eg.: black, white)
2. Emphasis (eg.: blue, green)
3. Deemphasized (grey)

From 8 ideas for Better Slides