Peter at 2011-09-22 18:39:25:
I've done 3-act structure on a 20-25 page shorts. I find my shorts work best, if I start with 1 solid scene with a theme. You can minor exposition the two other acts, and have just 1 solid Act, and technically it's still a whole story..
I've written a handful of 5 pager shorts that were pretty much just a 2nd Act or 3rd Act in a longer story.
Since I start more visually, I think of the awesome scene first, then I make sense of it from there.