duodecimo

Short definition from Wiktionary

Noun

1. 1
   A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.
2. 2
   A sheet or page of that size.
3. 3
   A book having pages of that size.