I've written and proofread a 60 page Game Design Document (GDDs) and Technical Design Document (TDDs) before for a 20 person project.
To fully complete the task, I would require more information about the game, such as controls and which language and/or engine it would be written in, etc.
I will write a fully inclusive GDD for you, including some technical specifications that will cover areas such as: overview, high concept, vision statement, audience, game world, gameplay overview, feature set, camera view, game interface, and sound. I can't predict how many pages it will take to complete, but I expect about 20-30.
The first step will be to form a customized GDD framework for your product.
The second step will be to populate the framework with your information. This may take some back and forth with you to get the details correct.
Finally, I will send it to you for your approval and to see if you'd like any changes.