This is a standard sheet of paper, folded in half. A good standard setting is a height of 8.5 inches and a width of 5.5 inches. Think about the ideal height and width of your book.

This is the size you want your book to be. Now you can begin to set up the layout for the pages of your book.

How to Change Pixels to Inches in InDesign From Picas to Pixels to Inches and more.