It’s probably a good idea to choose what size your book will be fairly early on. Things like images and tables are annoying to change up later (and in fact you might need completely different art for a different sized book). Google docs lets you change the “paper size” of the document. To do this, go to File->Page Setup and choose one of the paper sizes. At the time of writing, you can’t simply choose a custom size, but there’s a variety of popular sizes in there. If the one you want isn’t there, choose a larger size and set the margins so the text fills the space you expect.