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).
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