Which size should your book be?
Three things decide it — genre convention, word count, and print cost — in that order:
Match the shelf
Readers notice when a book is the “wrong” shape for its genre. Romance, thrillers and YA skew small (5 × 8 to 5.5 × 8.5). Literary, memoir and most non-fiction skew 6 × 9. Children's picture books are square or landscape. Look at the top twenty in your category and copy the majority.
Do the page-count maths
Bigger trim = fewer pages = lower print cost and thinner spine. Roughly, a 90,000-word novel runs about 360 pages at 5 × 8 and 300 at 6 × 9 with the same type settings. A short book can choose a small trim to look more substantial; a long one may need 6 × 9 to stay under KDP's 828-page cap.
Price the print cost
KDP's printing cost is a fixed fee plus a per-page charge (and colour pages cost several times black ink). Run your page count through KDP's royalty calculator at two trims before you commit: it often shows 6 × 9 earning more per copy on a long book.
Why this matters for marketing: your trim size sets the cover's proportions, and that proportion appears in every 3D mockup, every on-book ad image, and the comparison chart in your A+ content. A series that changes trim mid-run looks uneven on the shelf and in the ad. Pick once.
