Brandon Sanderson has confirmed that his new book, Oathbringer, the third volume in The Stormlight Archive, is very, very big.
In a Reddit update, Sanderson says that the novel is 25% longer than Words of Radiance, which came in at 400,000 words. That suggests that Oathbringer will be 500,000 words in length.
This would almost certainly make Oathbringer the second-longest fantasy novel of all time. The #1 spot is held by To Green Angel Tower (520,000 words), the third volume of Memory, Sorrow and Thorn by Tad Williams. The #2 spot is disputed, but probably goes to Ash: A Secret History, which clocks in between 493,000 and 500,000 (depending on if you count the notes or not). Two of Diana Gabaldon's Outlander novels also exceed 500,000 words, but the genre these novels occupy is highly questionable (since they are historical romances with a time travel element).
The Lord of the Rings, often cited as a very long epic fantasy novel, is a relatively breezy 470,000 words in length.
Oathbringer will be released, presumably in a very small font, in November this year.