How can I see how big my book's text will really look before I print it?

From version 1.9.22, wherever you choose the text size (Book design on the book's page, or "Print & text size" in New Book), a page of your book is drawn to scale with the chosen trim, margins and size. Press "See it at actual size" to show the page at its real printed size on your screen: hold the screen at reading distance to judge it the way a reader will. Press Close or Escape to go back.