As someone who's judged in the past. And judged the first round (which is the one who the majority of the Preppers enter), shorter isn' necessarily better. But don't try to lengthen it thinking that will make you a winner either.

Like everyone said, the only requirement is the 15 minutes of prep time. But make it interesting. Judges aren't required to keep reading if they aren't interested. And trust me, I've tried to read and review all the entries; its just not possible!

So make it interesting, make it pertinent. And most of all have fun!