* 25km (25 kilometers) is about 15.5 miles, far too large for a book.
* 25m (25 meters) is about 82 feet, also far too large for a book.
* 25mm (25 millimeters) is about 1 inch, a realistic thickness for a paperback book.
Think about how thick a typical paperback book feels in your hand. 25mm is a good approximation.