What page size (trim) should my book be?
Updated
6×9 in is the standard for novels over ~60,000 words and most nonfiction — more words per page, lower print cost per copy. 5×8 or 5.25×8 suit shorter fiction and novellas; 5.5×8.5 is a comfortable middle. Match your genre's shelf and, for a series, match your earlier books.
Trim drives economics: print-on-demand charges by page count, and a 6×9 page holds roughly 25–30% more words than 5×8, so long books cost less per copy at 6×9.
Trim also signals genre: compact trims read as fiction paperbacks; 6×9 reads as trade paperback. Pick the shelf you want to sit on.
Because the choice is one click in automated tools, the practical answer is to render both and compare page counts and feel before committing.