The Collaborative International Dictionary
Sixteenmo
Sixteenmo \Six*teen"mo\, n.; pl. Sixteenmos. See Sextodecimo.
Douglas Harper's Etymology Dictionary
sixteenmo
"book printed on sheets of 16 leaves," 1847, from an English reading of the printers' Latin abbreviation 16-mo, representing sexto decimo "sixteen."
Wiktionary
sixteenmo
n. sextodecimo (as a paper size in printing).