A book the size of one fourth of a sheet of printing paper.
I love this post wherein fellow WordPress.com blogger Jonathan Kearns illustrates various book sizes. You should really click over there and enjoy the entire thing, complete with photos, but his description of the quarto size is too delightful not to share:
“Next up is the quarto (4to), somewhat larger and heavier than the octavo, for a variety of reasons this size is beloved of eighteenth and nineteenth century creators of travel books; everything from Cook’s Voyages to Franklin’s Journey to the Polar Seas can be found in this size. The book being held by the scary man is in fact John Barrow’s copy of the Travels of Marco Polo, thus adding weight to my somewhat tenuous point. The quarto is beloved of people who want to produce books full of elaborate folding maps and charts. And God books. frequently seen in quarto is God.”
A small quarto book of four plays I’d never heard of before.