Page size feels like the first decision and it is one of the least consequential, because for most books the page will never be paper. The three families of size exist for three different destinations, and once you know where the book is going the choice mostly makes itself.
The three families
| Family | Examples | Made for |
|---|---|---|
| ISO | A4 (210 × 297mm), A5 (148 × 210mm) | Office paper nearly everywhere outside North America |
| North American | Letter (8.5 × 11in), Half Letter (5.5 × 8.5in) | Office paper in the US and Canada |
| Trim sizes | 6 × 9in, 5.5 × 8.5in, 5 × 8in | Bound books from a printer or print-on-demand service |
The first two families are what home and office printers are loaded with. The third is what a printed book actually is: a trim size is the finished, cut dimensions of a bound page, and a paperback from a shelf is almost always one of a dozen standard ones. A 6 × 9 novel is not a page size any printer in a home holds. It is what you get when a print service prints on larger stock and trims it.
What screens want
Nothing in particular. A PDF on a laptop is scaled to the window, and a PDF on a tablet is scaled to the screen. The page's physical size is a number in the file that the viewer uses to work out proportions, and beyond that it is invisible. What the reader sees is the *ratio* of the page and the relative size of the type on it.
This is why the page size question is less important than it looks: for the majority of PDFs, which are read on screens, A4 and Letter and 6 × 9 are three rectangles of slightly different proportions and nothing more. The reader will never know which one you chose unless they print it.
A phone is the exception and the reason you probably also want an EPUB. A fixed page of any size is a poor experience at phone width. No choice of page size fixes that; only a reflowable format does.
What home printers want
The paper that is in them. Outside the US and Canada that is A4, essentially without exception. Inside them it is Letter. If the book will be printed by its readers — a workbook, a manual, a guide someone wants on their desk — then the page size should be one of those two, and the choice between them is a choice about where the readers are.
A4 is the reasonable default because it covers most of the world's printers and prints acceptably on the rest. A Letter-sized page sent to an A4 printer loses a little at the sides unless scaled; an A4 page sent to a Letter printer loses a little at the foot unless scaled. Both are recoverable with the "fit to page" box that every print dialog has, and neither is the disaster that sending a 6 × 9 page to either printer is — which prints a small rectangle in the middle of a sheet with margins the width of a hand.
When a trim size matters
When the book is going to be bound. A print-on-demand service or a printer needs the file at the finished page size, because their whole process — imposition, printing on large sheets, folding, trimming — is built around it. Send them A4 for a book you want at 6 × 9 and either they reject it or they scale it, and scaled type is the wrong size on the page.
- 6 × 9 in (152 × 229mm) — the most common trade paperback size in the US, and the default most services suggest. Non-fiction, longer fiction.
- 5.5 × 8.5 in (140 × 216mm) — a slightly smaller, more hand-sized paperback. Shorter fiction, poetry, essays.
- 5 × 8 in (127 × 203mm) — mass-market territory. Novels.
- 8.5 × 11 in and 8.27 × 11.69 in — Letter and A4 as trim sizes. Workbooks, manuals, textbooks, anything with tables or worksheets.
That last row is the one people miss. Amazon KDP lists 8.27 × 11.69 inches — which is A4 — among its paperback trim sizes, alongside Letter. So an A4 PDF is not a screen-only decision; it can go to print as a large-format paperback without being re-laid, provided the margins meet the service's minimums and the file is print-ready. For a workbook or a technical manual that is often exactly the right format. For a novel it is not, because nobody wants to hold a novel the size of a magazine.
Margins change the feel more than size does
Here is the part that makes the decision less fraught. Two books at the same page size can look completely different, and two books at different sizes can look nearly identical, because what the reader perceives is the text block — its width, its position on the page, and the white space around it — not the edge of the paper.
An A4 page with 40-point margins is a wide, dense sheet that reads like a report. The same A4 page with 100-point margins and 11pt type has a text block about the width of a 6 × 9 page's, sitting in generous white, and it reads like a book. Same paper, different object. The measure — the characters per line — is what changed, and that is the number that decides readability, not the dimensions of the sheet.
| Page | Margins | Text width | Feels like |
|---|---|---|---|
| A4 | 40pt | 515pt | A report |
| A4 | 100pt | 395pt | A large-format book |
| 6 × 9in | 54pt | 324pt | A paperback |
| Letter | 90pt | 432pt | A textbook |
So if you are producing an A4 PDF and worrying that it will not feel like a book: the fix is in the margins, and it costs nothing.
A way to decide
- Screen only. A4. Set the margins wide. Also make an EPUB for phones.
- Screen, and readers may print it at home. A4, unless your readers are almost all in North America, in which case Letter. Either prints acceptably on the other with scale-to-fit.
- Bound paperback, prose. A trim size — 6 × 9 for most non-fiction, 5.5 × 8.5 or 5 × 8 for fiction — and a file laid out at that size with mirrored inner margins.
- Bound paperback, workbook or manual. A4 or Letter as the trim size. The service accepts them; the extra width is what tables and worksheets need.
bukpress exports A4, with margins in points, and that covers the first, second and fourth cases directly. What it does not do is lay out a 6 × 9 novel for a press, and it is better to know that before starting than after. The sample book shows what an A4 page looks like when the margins are doing their job, and what Amazon KDP actually wants from your file covers the rest of the print path, including their margin minimums for each trim size.