One example “English Syntitholgy, by James Brown, 12mo., Boston and Philadelphia 1842” among several at AE Monthly - the Magazine for Book Collectors & Booksellers.

16mo. seems to be most common e.g. “a 16mo. pamphlet” and “A small brochure, a 16mo.,”

16mo stands for “sextidecimo”, which means that the original paper sheet was folded into 16ths before binding and cutting.  The letters at the end vary depending on the number: duodecimo is 12mo, but octavo is 8vo and quarto is 4to.  If the original sheets were folded into 2 leaves (4 pages of printing if printed on both sides) that’s a folio.  The full-size sheets vary in size (especially historically) but are generally too large to make a practical-sized book if simply bound together flat without folding.  Most hardback books today, 8-10 inches tall, are octavos (or considered such, since the terminology now refers more to the sizes than to the actual folding of paper sheets).  16mo would be about 6-7 inches tall and 4 inches wide.

A term like “a 16mo. pamphlet” doesn’t actually define the size of anything; as Dr. Techie makes clear, it just tells you the number of times the whole sheet was folded after printing. In the days before DIN and (later) ISO, the size of a printed page was defined by two terms: the size of the full sheet of paper, of which there were literally dozens in Europe and America (Crown, Demy, Large Post, Foolscap, etc., are names of paper sizes that go back hundreds of years), followed by the number of folds (folio, 4o., 8vo., etc.). In the US today,a more utilitarian nomenclature is used for different sized sheets of paper: Bond, Letter, etc. (these are terms which define to some extent the type of paper), followed by the size of the sheet in inches (22 x 24, etc.).

This site has a useful table listing names, folds, sheets, pages and abbreviations.

Folio - 1 fold, 2 sheets, 4 pages
Quarto - 2, 4, 8
Sexto - 3, 6, 12
Octavo - 3, 8, 16
Duodecimo - 4, 12, 24
Sextodecimo - 4, 16, 32

It also has tables for cut writing paper sizes and uncut writing paper sizes, with names I hadn’t come across before. For instance in the cut, Albert (4.0 x 6.0 in), Duchess (4.5 x 6.0 in) and Duke (5.5 x 7.0 in). It’s a pity to have lost such terms but such is the way of the world.