Lord Alexander's Cipher; or, the Bridekirk Behemoth will be bigger than Gone with the Wind. Not in word count - it's nowhere near the length of Margaret Mitchell's monster - but in terms of its chances for best-sellerdom.
Language Log has pointed me at the Lulu Book Title Analyzer, which uses an algorithm based on a book's title to forecast its chances. LAC;o,tBB scores 55.4%, which more than GWTW (44.2%) and considerably more than The Da Vinci Code (10.2%).
I fall short, however of the heights reached by the illustrious Jane Austen. Pride and Prejudice, Persuasion and Sense and Sensibility apparently each have a 59.3% chance of being best-sellers, with Emma only a little way behind at 45.6%. That young lady will go far.