6 × 9 inches is the safe default for most novels and non-fiction. 5 × 8 or 5.5 × 8.5 suit shorter novels and look like trade paperbacks in shops. Pick one before formatting; changing later means redoing the interior and the cover.
Rules of thumb
Smaller trims mean more pages, which raises the print cost and widens the spine. Larger trims mean fewer pages.
Look at the books your readers already buy in your genre and match them.
Check the list
KDP lists every supported size and which ones allow hardcover and expanded distribution.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.