The most popular book size on Amazon KDP is 6" x 9" (15.24 x 22.86 cm), which is Amazon's default selection and the industry standard for novels, trade paperbacks, and standard non-fiction.
Most Popular Trim Sizes by Genre
6" x 9" (Standard & Hardcover): Best for standard novels, memoirs, self-help books, and general non-fiction. This is the default choice in KDP's print setup.
5.5" x 8.5" (Compact / Trade): Great for literary fiction, compact non-fiction, and books that feel a bit more traditional or giftable.
5" x 8" (Small Novel): Ideal for smaller genre fiction, novellas, and pocket-friendly paperbacks.
8.5" x 11" (US Letter): The standard choice for coloring books, workbooks, puzzle books, and textbooks.
8.5" x 8.5" (Square): The most popular format for children's picture books and illustrated or design-heavy books.
7" x 10" or 6.14" x 9.21": Common for academic texts, manuals, or larger format non-fiction with images.