I suggest you assign yourself a trial period to test the idea, and let everyone know you want their feedback during that time. That might excuse you a bit and get more people on board. “For the Q1 2010, Cindy is participating in this experiment to reduce digital clutter. [Link]”

Five sentences is still a lot. I’d like to see some real examples of messages which still require a longer response, but not a phone call. If more than two or three people are involved (when phone calls are near impossible to schedule) then the message should never be more than 5 sentences. If only two or three people are in the conversation, a phone call may take just as long as composing the email.

Although, it’s nice to have searchable records of conversation, but most people don’t have their email set up for that anyway.

Google Wave, Wikis, and blog posts are good forums for long-form, but asynchronous discussion.

That being said, I wouldn’t want to constrict the length of certain email relationships. It’s where some exchange their most catalytic questions and responses.