So I just encountered this website, which purports to list the word counts of several major fantasy books. I can't vouch for the accuracy of the site, since the blogger seems to assume that a typical page equals 400 words, and there's no telling if they are using the hardcover or paperback versions of these fantasy novels, which makes a huge difference. Still, the list is interesting.
Ole' Tollers comes in at a modest 500,000 words, making The Lord of the Rings the short story of the bunch. The first two Covenant trilogies by Stephen R. Donaldson also come it around this figure.
George R. R. Martin's The Song of Ice and Fire is a "mere" 1,740,000 words -- and I say "mere" because several series have word counts that extend to the millions. Janny Wurt's The Wars of Light and Shadow isjust a tick over 2 million words, and Dorothy Dunnett's Lymond + Niccolò just a tick under. (Never heard of either writer, btw). All ten of Donaldson's Covenant books are about 2,062,000 words. The doorstoppers properly begin with Stephen Erickson's Malazan books (I hated the first two) at 3,274,000 words and Diana Gabaldon's Outlander 3,227,000 words.
But the grand prize would probably have to go to Robert Jordan's The Wheel of Time series. That's at 3,263,000 words technically less than Erickson, but I feel that we should add another 1,000,000 words for the three continuation books by Brandon Sanderson. So, about 4.26 million words total for the whole series.