Choosing the right trim size
Trim size is your book's physical dimensions. Amazon KDP offers many, but a few dominate each genre: 5×8 for novels, 6×9 for non-fiction, square 8.5×8.5 for children's books, and 8.5×11 for workbooks. Matching the genre standard makes your book look professional next to competitors.
Why gutter margins grow with page count
Thicker books need a wider inside (gutter) margin so text isn't swallowed by the spine. This guide shows KDP's recommended minimum gutter for your page count — always add a little extra for comfort.