The Collaborative International Dictionary of English v.0.48:

Quarto \Quar"to\, n.; pl. Quartos.
   Originally, a book of the size of the fourth of sheet of
   printing paper; a size leaves; in present usage, a book of a
   square or nearly square form, and usually of large size.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Quarto \Quar"to\, a. [L. in quarto in fourth, from quartus the
   fourth: cf. F. (in) quarto. See Quart.]
   Having four leaves to the sheet; of the form or size of a
   quarto.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quarto
    n 1: the size of a book whose pages are made by folding a sheet
         of paper twice to form four leaves [syn: quarto, 4to]