Length is an interesting thing with manifestos. Most manifestos on this site are under 200 words because they come from TV commercials, but some of the best pieces push 400 and up.
The key is to never, ever pad your manifesto. Only make it as long as you can be interesting. But if you can hold attention, go for it. Almost nobody realizes the Johnnie Walker piece is over six minutes.
Top 10
BBH “The Power of Difference”
Cadillac "The Penalty of Leadership”
DDB "Let’s Tell the Truth”
Johnnie Walker "The Man Who Walked Around the World”
Miller "This Is the High Life”
Nike “A Message from Nike Running”
Nike “Dennis Rodman, American Hero.”
Papercut “This Is Not Your Life”
Rainforest Alliance “Follow the Frog"
Ram “So God Made a Farmer”