I'm thinking 80 pages.
About 4 pages for all that stuff at the front..titles, publishing details, contents
Followed by an 8-page Chapter 1, giving a rough overview of the world, who the characters are, what they're aiming towards, what they tend to do to get there, and what's stopping them.
A 16-page Chapter 2, describing the process of setting up play, defining the realm where the stories will unfold, the characters (main cast and supporting roles) who will be involved in those stories, and the specific places and items that will be important within the stories.
A 30-page Chapter 3, describing how mundane actions are perform (and separately how magical actions are performed), followed by how those actions build up into complete narratives. Since the actions are freeform, but players can define specific actions/spells that have been practiced to gain a more consistent outcome, the last half of this chapter will consist of sample actions and spells that might be picked up by the characters, or used as templates to develop new abilities/spells.
A 14-page Chapter 4, filled with sample supporting cast (NPCs), locations and items ready to slot into any realm where stories might unfold. This chapter is designed to help players and GM get into the action quicker by providing prompts and story fragments that might fill in some gaps or add flavour.
An 8-page Appendix, filled with character sheets (for main cast and supporting), relationship sheets, and other play aides. Finishing off with an index.
The whole thing hand written and illustrated.
I'll probably have to type it up first though, and get feedback from people before I get stuck into the full hand-rendered project.