I''m agreeing with Felonius. Design Docs are a must have on projects of any decent size - especially with other people. I would personally split Felionius'' 300 page monster into separate docs (I don''t think things like the code structure belong in the DD), such as the Technical Design Doc, and the Art Bible. Although I know the Age of Empires DD was about 300 pages. But then again, RTS get very complicated very fast when detailing their systems, and it was a large team project so they needed to spell out the design in exact detail.

And its important to keep the DD a fluid and ongoing thing. Our DD is sitting in the CVS tree right in with the code (actually in a doc directory along with some of the technical documentation), and whenever I think of something that needs to be added or changed, its a simple checkout, edit, submit and the whole team has access again.

Interestingly enough I just posted a request for feedback on creating what would be termed a design document on this thread in the forum above.

http://www.gamedev.net/community/forums/topic.asp?topic_id=156322

I am looking for ideas, preferably tried ones, on the facets of an MMORPG game in outline form, which our group will discuss and adapt to our project to then use in fleshing out the concepts for the game.

While I agree a design doc may not be necessary, the larger and more complex the scope of the game the more critical it becomes to pre-check your bases being covered. Having at least the concepts outlined in logical form certainly would not hurt a project, and would give the Project Manager a blueprint to direct the modular creation and assembly of the whole.