6x9 is a popular size for indie publishers these days. At almost exactly half the page area of 8.5x11, it’ll double the number of pages in your book. This can be good if you have a book on the shorter end of things, to make it feel more substantial. At the same time, if this pushes your page count too high, it’ll make the book feel like you’re holding a pocket dictionary. Personally, I think 200-300 pages is ideal for most RPG books… it makes it feel like you’re getting a good amount of text for your money, without feeling intimidating like a college textbook.