Peter F. Hamilton's The Naked God is almost certainly the biggest single SF novel ever published at 1,150 pages in hardcover (and 1,290-odd pages in paperback only possible by reducing the font size and squeezing out the margins). Plus it's only the third book in a trilogy.
For fantasy, it's a bit more oblique. The Lord of the Rings is up there at c. 1,100 pages even in hardcover and tradeback editions. A Storm of Swords is 1,200 pages in the US mmpb edition, which is pretty huge. But I think the winner is Tad Williams' To Green Angel Tower, which was just short of 1,200 pages in hardcover and had to be split into two 800-page mmpbs for the paperback release. His Otherland: Sea of Silver Light isn't that much shorter, although the mmpbs were squeezed into single 1,200-page volumes.