From 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. […] 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. […] 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 you’re optimal font size.

Smart stuff.