that depends on the book Format itself. You can use the Resize Tool to fit your Pages into that Format.
In General 6x9 is one of the Formats. But you also can resize it to 5x8 or 4,25x6,87. As you can see the , the ratio of the long edge to the narrow edge is always the same. The best thing to do is to ask the publisher which format you want. Hope it helps.