The Collaborative International Dictionary of English v.0.48:

Sextodecimo \Sex`to*dec"i*mo\, a. [L. sextus-decimus the
   sixteenth; sextus the sixth (fr. sex six) + decimus the
   tenth, from decem ten. See -mo.]
   Having sixteen leaves to a sheet; of, or equal to, the size
   of one fold of a sheet of printing paper when folded so as to
   make sixteen leaves, or thirty-two pages; as, a sextodecimo
   volume.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sextodecimo \Sex`to*dec"imo\, n.; pl. Sextodecimos.
   A book composed of sheets each of which is folded into
   sixteen leaves; hence, indicating, more or less definitely, a
   size of a book; -- usually written 16mo, or 16[deg].
   [1913 Webster]