Trim size is one of those decisions new self-publishers make on a whim — usually by picking whatever the print-on-demand platform defaults to — without realising it's quietly signalling genre expectations to every reader who picks the book up, before they've read a word of it.
5×8 and 5.25×8 are the closest print-on-demand equivalent to the old mass-market paperback — compact, pocketable, and the size most genre fiction readers associate with thrillers, horror, and commercial fiction generally. Every one of my own novels, from Billy Witch to the Legions of Honour series, ships in this range, because it's what the target reader's hand expects to be holding.
6×9 is the standard trade paperback size — larger, roomier margins, and the size most readers unconsciously associate with literary fiction, nonfiction, and textbooks. It's not wrong for a novel, but it changes the impression before the first page, and it interacts badly with page count: a 60,000-word novel in 6×9 prints thin enough to look insubstantial on a shelf, where the same manuscript in 5×8 fills out properly.
That interaction between trim size and page count is the part most new authors miss entirely. The same word count produces a very different number of printed pages depending on the trim size, because a smaller page fits less text per page. A book that feels appropriately weighty at 5×8 might run to half as many pages at 6×9, and spine width — which is calculated from page count, not word count — follows directly from that choice. Get the trim size wrong for your book's length, and you either end up with an oddly thin paperback or an oddly bloated one.
The practical fix: pick your trim size based on genre convention first — look at five bestselling books in your exact category and note what size the physical copies are, if you can find them — then let your actual manuscript length confirm whether it'll fill that size properly. If it's on the short side for the trim size you want, that's worth knowing before you finalise a cover, not after.
Once the trim size is decided, the actual spine-width maths — which depends on page count and paper stock as much as trim size — is exactly the tedious, error-prone calculation I built Spinework to handle automatically, live, the moment you enter your numbers.