QUARTO is a 6 letter word that starts with Q.

1. \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.
   
   

2. \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.