Before writing a book, I configure my Word doc to the Print on Demand format. This offers the look of a paperback. The added benefit is knowing exactly how long your book will be when printed.
There are many sizes you can choose from. I found 5½” by 8½” to be the optimal size for self-publishing. I will reformat your Word document using Amazon’s Print on Demand standards. When publishing, the file must be saved as an Adobe PDF.
Here is a screenshot of my book from the Amazon Previewer. The hardest part is getting the margins to align with the gutter (the part that gets sucked into the spine). There’s also a trick to starting the page numbering when the story begins, not in the front section.