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]