It is 20 volumes, the mother of all dictionaries. People don't understand the name of the 2-volume Shorter Oxford English Dictionary, because they cannot imagine anything larger. Well here it is:
It is so big because it defines not only by prescription ("this means such-and-such"), but also by example. It gives heaps of quotations that illustrate how the word has been used over the years. It even gives the source of the quotations. The entry for the word "set" is over 18 pages long! It's wonderful for bookworms.
Oxford University Press has a website devoted to the OED, as it is often called.