I seem to find that the standard book size is six inches wide and nine inches tall 6 X 9. I've read that your book will probably be cheaper to print, easier to read, and easier to sell if going with the 6 X 9. One reason because it will fit on most bookstore shelves.
Larger books can be harder to hold, carry, or put on a shelf.
The cover of a 6 x9 book is larger than a 5.5 x 8.5 book. The cover is your advertising space so you want the best available book size.
I've printed two books with this format and it seems to work well. Go to file and page-set up and use these settings below for a 6 x 9 paperback novel.
Here are some images to help you set your M-Word so you know how many chapters you have and at what page you are on in your current Novel.
First set your Margins.