This is a rule suggested by Guy Kawasaki.
Before there is an epidemic of Ménière’s in the venture capital community, I am trying to evangelize the 10/20/30 Rule of PowerPoint. It’s quite simple: a PowerPoint presentation should have ten slides, last no more than twenty minutes, and contain no font smaller than thirty points. While I’m in the venture capital business, this rule is applicable for any presentation to reach agreement: for example, raising capital, making a sale, forming a partnership, etc.
Read more on Guy Kawasaki’s blog.