From Wikipedia, if they are correct, here is a list of the longest novels:

Henry Darger, The Story of the Vivian Girls, in What is known as the Realms of the Unreal, of the Glandeco-Angelinnian War Storm, Caused by the Child Slave Rebellion-Illustrated fantasy novel manuscript typed single-spaced with 15,145 pages in 10 volumes. Discovered after Darger's death, the manuscript has never been published. The total number of words has been estimated; some believe this might be the longest novel ever written. The most conservative guess will put this in the million-word realm, possibly into tens of millions.

Madeleine de Scudéry, Artamène-Published in ten volumes from 1649–53. It contains 2.1 million words. Available online (in French only) at Artamene.org.

Marcel Proust, À la recherche du temps perdu-Longest conventionally-read novel. 9,609,000 characters, nearly 1.5 million words. Holds the Guinness Book of Records title as Longest Novel. Published in 13 volumes from 1913 to 1927. English translation is titled Remembrance of Things Past or In Search of Lost Time.

L. Ron Hubbard, Mission Earth-1.2 million word. Published 1986 to 1988 as ten volumes.

Madison Cooper, Sironia, Texas-Published 1952. 1731 pages, originally published in two volumes. Estimated at 1,100,000 words.

Samuel Richardson, Clarissa-Published 1748. First edition contains about 969,000 words. Third edition contains some 5,500,000 characters, over one million words. Published in nine volumes. Conventionally regarded as the longest novel in the English language.

Anthony Powell, A Dance to the Music of Time-Published from 1951 to 1975, as a partial tribute to Proust. Published in 12 volumes, it is sometimes regarded as a novel sequence. At over 1 million words, it is a contender for the longest English-language novel ever written.

Xavier Herbert, Poor Fellow My Country-Published 1975. At 1463 pages and 850,000 words, it is the longest Australian novel.

Marguerite Young, Miss MacIntosh, My Darling-Published 1965. Contains "714,859 words on 1,198 closely printed pages."

Courtney Thomas, Walls of Phantoms-Published 2007. Coming in at 629,858 words that fill 1320 pages, Walls of Phantoms - which retells the Odyssey in a modern setting - is the third-longest single-volume novel written, and perhaps the longest by an African-American.

Alexandre Dumas, père, Le Vicomte de Bragelonne, ou Dix ans plus tard-Written in 1847. When published in English it was usually split into three parts The Vicomte de Bragelonne, Louise de la Valliere and The Man in the Iron Mask. Estimated at some 626,000 French words.

Alexandre Dumas, père, The Count of Monte Cristo-Completed in 1844, and published in serialized form from 1844-46, at roughly 1460 pages, it is Dumas' longest single volume novel.

Robert Musil, The Man Without Qualities-Published in three parts, two in 1930, the third after Musil's death in 1941. Unfinished at over 1700 pages.

Vikram Seth, A Suitable Boy-Published 1993. 1488 pages softcover. 591,552 words. The longest conventional novel in English since Clarissa, and officially[citation needed] the longest novel in the English language published in a single volume.

Ayn Rand, Atlas Shrugged-Published 1957. 565,223 words. 1274 pages.

Leo Tolstoy, War and Peace-Published from 1865 to 1869. Original text has some 460,000 Russian and French words. English translation contains over 560,000 words and over 3.1 million characters; typically over 1400 pages as a paperback.

Carl Sandburg, Remembrance Rock-Published 1948. 532,030 words.

Victor Hugo, Les Misérables-Published 1862. Nearly 513,000 French words. Unabridged French edition (ISBN 2070102645) 1779 pages (including essays).