Book sizes in printing terms are called trim sizes. In the last stage of production a book is trimmed, which is how it gets its name. There isn’t a set size that you should choose, but there are a few things to take into consideration when picking your trim size.
Most authors will be using a print-on-demand service like CreateSpace or IngramSpark. Both have a large list of trim sizes to choose from. So, how do you know which one to choose?
First, break it down based on what type of book you have written. Then look at your word count and determine how many pages your book might be*. Once you know that, look at the cost of producing the different sized books and make your decision from there. Also, take into account the page color, as that can limit your selection as well.