duodecimo

noun

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 A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.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

   “In recent years, I have taken to reading while walking. I believe the Romantics were guilty of such behavior; a duodecimo was small enough to fit into a jacket pocket.”