Set page size (trim size) and margins for print

What you'll do: choose the finished size of your printed pages and the blank space around the text. These settings are for the print PDF — they don't affect the e-book.

Both live on the Design step. See What is trim size? and Margins, the gutter, recto & verso.

Choose a trim size

The Paper trim size section on the Design screen

Go to Design and open Paper trim size.
Switch units between inches and mm if you like.
Pick a size. Common novel sizes are listed, and each shows which retailers support it (KDP, IngramSpark, Lulu).

A trim size of 5″ × 8″ or 5.5″ × 8.5″ is typical for fiction.

Set the margins

The Layout & margins section on the Design screen

Open Layout & margins:

Set Top, Bottom, Inside and Outside margins. The Inside margin is the gutter next to the spine — make it a bit larger so text isn't swallowed by the binding.
Turn Justified on for flush edges, and Hyphens on to keep the spacing even.
Choose where chapters begin (left page, right page, or either) and whether parts and chapters get their own page.

Preview a Print device (see Preview on devices) to check it looks right, then make the file in Make a print-ready PDF.