After I formatted the inside of my book (see the blog post), I tried out different book sizes. I did this while still using MS Word.
A small book of 4X6 or 5X8 created a thick book that I thought would be difficult to handle. It also could be intimidating to read by some people in my genre. More importantly, having so many pages would be expensive to print. Too large of a book, such as 8X10, had fewer pages making it less expensive to print. But it was big and awkward and did not have enough spine to show the title. I also thought the readers might think they were not getting value with a thin book.
For a 68,000-word novel, I chose a 6X9 book, which is popular in my genre. With margins set wide and 1.5 inch spacing between lines, this produced a book of 335 pages and making about 200 words per page. I could have reduced the margins or spacing and put more words on each page, but what I had gave a lot of white space. Based on other books in my genre, I thought this looked more appealing to the reader.
Next, I went to the end of each chapter. If there were one to three lines left, I reduced wording in that chapter which reduced the lines and saved the printing of a mostly blank page.
When done, I saved the Word document as an Adobe file. This mostly locked in the design posing fewer problems when transferring to KDP software. MS Word software can be proprietary and complex causing unwanted format changes when transferring to another software.
I uploaded this Adobe document into KDP.
KDP did make some additional changes to fit the document into their system. However, after reviewing each page of the final document, my novel looked very much like what I had uploaded.