The Collaborative International Dictionary of English v.0.48:

Decimosexto \Dec`i*mo*sex"to\, n. [Prop., in sixteenth; fr. L.
   decimus tenth + sextus sixth.]
   A book consisting of sheets, each of which is folded into
   sixteen leaves; hence, indicating, more or less definitely, a
   size of book; -- usually written 16mo or 16[deg].
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decimosexto \Dec`i*mo*sex"to\, a.
   Having sixteen leaves to a sheet; as, a decimosexto form,
   book, leaf, size.
   [1913 Webster]