Guy Kawasaki, in one of his classic posts, put together a set of guidelines for using PowerPoint within presentations. His rule, if followed, would keep presentations focused, interesting and make them effective.
Here is Guy’s rule:
A PowerPoint presentation should have ten slides, last no more than twenty minutes, and contain no font smaller than thirty points.Here is Guy's rule unpacked from his original post....
Ten slides. Ten is the optimal number of slides in a PowerPoint presentation because a normal human being cannot comprehend more than ten concepts in a meeting
Twenty minutes. You should give your ten slides in twenty minutes. Sure, you have an hour time slot, but you’re using a Windows laptop, so it will take forty minutes to make it work with the projector. Even if setup goes perfectly, people will arrive late and have to leave early. In a perfect world, you give your pitch in twenty minutes, and you have forty minutes left for discussion.
Thirty-point font. The majority of the presentations that I see have text in a ten point font. As much text as possible is jammed into the slide, and then the presenter reads it. However, as soon as the audience figures out that you’re reading the text, it reads ahead of you because it can read faster than you can speak. The result is that you and the audience are out of synch.
The reason people use a small font is twofold: first, that they don’t know their material well enough; second, they think that more text is more convincing. Total bozosity. Force yourself to use no font smaller than thirty points. I guarantee it will make your presentations better because it requires you to find the most salient points and to know how to explain them well. If “thirty points,” is too dogmatic, the I offer you an algorithm: find out the age of the oldest person in your audience and divide it by two. That’s your optimal font size.
Read the rest of Guy's 10/20/30 rule here