The word "book size" refers to the physical dimensions of a book, including its height, width, and thickness. However, there are many synonyms for this term that one might use instead, depending on the context. For example, one could refer to the book's "format" or "layout," which encompasses not just the physical size but also the design and structure of the pages. Another option might be "page size," which refers specifically to the dimensions of the individual pages within the book. "Dimensions" or "measurements" are also suitable alternatives for discussing the size of a book, although they may be somewhat more generic than the other options.