Choosing a trim size
Trim size is the finished width × height of your printed book. Bookshaper offers 17 trim sizes, each marked with whether KDP's and IngramSpark's paperback specs list it — an advisory, since platform catalogs change, so confirm with your printer. They run from mass-market and the common 5×8, 5.5×8.5, and 6×9 novel sizes to trade sizes like 6.14×9.21, square formats, and the larger nonfiction trims — so you can match the convention for your genre and printer instead of forcing your book into one of a handful of presets.
Trim size lives with your book's page geometry in Project Settings. Set it once; the active style template lays out every page against it, and the print preview and export both compile at that exact size.
Not sure which to pick? Design My Interior recommends a trim size alongside its template, ornament, and font suggestions, with a short rationale for why it fits your genre.
The page-count-aware margin advisory
The inner (gutter) margin is the one most authors get wrong. Print-on-demand services require a larger gutter as the book gets thicker, because a fat spine swallows more of the inner edge — KDP's minimum inner margin scales from about 0.375″ for a short book up to 0.875″ and beyond for long ones. Set it too small and the printer rejects the file or text disappears into the binding.
Bookshaper watches your real compiled page count and surfaces a non-blocking advisory when your inner margin is below the recommended minimum for that thickness. It's guidance, not a hard stop — you stay in control — but it means you find out while you're setting margins, not after a rejected upload.
Verify before you export
The margin advisory pairs with two later checks. Print-Ready Check compiles the real print PDF and flags any line that intrudes into the gutter, and the pre-export preflight report verifies your gutter against your actual page count one last time before you generate the file. Together they mean a trim-and-margin mistake is caught at the point you make it, at layout time, and again at export.