8.5x11 is the traditional “D&D” size, but please don’t make your book full page-width. Such long lines are hard for humans to read. Instead, split your book into columns. In the last year or so, Google has rolled out column support for Docs. To change the whole document, make sure no text is selected, then go to Format->Columns and choose the one two or three column layout. Most 8.5x11 RPG books these days are two columns, but 1st and 2nd edition D&D used three columns, so if you want a really old school look, try that out. To get some text to span columns, select that text and go to Format->Columns again and choose single column. Thus you can have your chapter or major section titles span the full page. This is also a great way to break both columns in the middle of the page and still have text below them.