Three formats, one bindery. Every booklet here is saddle stitched unless you ask otherwise, printed in signatures on shared sheets, and trimmed on three edges after binding, which is why the page count has to be a multiple of four. Filter by how the finished book will actually be read.

8.5″ × 11″ page: catalogues, manuals and reports.

5.5″ × 8.5″ page. Read in the hand, cheaper to mail.

Your trim, extent and binding. Standard sizes or binding will not do.
Start with where the booklet is read rather than what goes in it. Anything consulted at a desk (a catalogue, a manual, a report with tables or diagrams) wants the full letter page, because technical content cramps badly at smaller trims and readers expect that size for reference material. Anything read in the hand, carried, or posted in volume is better at half-letter: it holds open one-handed, which a letter-size booklet does not, and it costs noticeably less to mail.
Two constraints are not negotiable and are worth knowing before you plan content. The extent has to be a multiple of four, because one folded sheet makes four pages. Asking for eighteen means paying for twenty with two blanks. And saddle stitching runs out at roughly sixty-four pages depending on stock bulk, past which the spine bulges and the outer pages protrude. If you are near either limit, ask for a dummy in the real stocks before committing to a run.