Definition of 12mo:
(n) :
(printing) A book size using duodecimo-sized pages.
(n) :
(paper) Abbreviation of duodecimo, a page size (5"-5.5" x 7.125"-7.5") [(paper, printing) A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.]