Definition
Trim size
Trim size is the finished width × height of a printed book's pages — 6×9 in is the most common self-publishing trim. Your interior PDF's pages must match the trim exactly.
The name comes from printing: pages are printed on larger sheets and trimmed to final size. Print-on-demand platforms like KDP ask you to choose a trim, then require the interior PDF's page size to equal it precisely — a Letter-size export of a '6×9' book is the classic rejection. Trim choice also drives cost (bigger pages hold more words, so fewer pages) and genre feel (compact trims read as fiction paperbacks).
Related tools & guides
Measure yours
Drop an interior PDF into the free KDP checker: it detects the trim, measures the inside margin on every page against the gutter your page count needs, and names the pages that fall short.