How do I change my book's trim size (page size) in BookyAI?

Open your book and find the Book design card. Below the design templates you'll see a Trim size selector with eight standard print sizes: 5 × 8 in, 5.25 × 8 in, 5.5 × 8.5 in, A5, 6 × 9 in (the default), 7 × 10 in, 8.5 × 11 in and A4. Pick one and it applies immediately to your next export: the PDF pages, the covers and the Word document all use exactly that page size, with proper print margins for that trim, and images scale to fit the page. Your text is never changed by the trim choice — only the layout. EPUB and Kindle exports are not affected because e-books reflow to fit each reader's screen; that is by design and required by the e-book stores.