GONE WITH THE WIND is 423,575 words. A novella is about 50K words.
This is an inaccurate comment. 423,000 words is not a typical novel length. The typical novel length is 75,000--100,000 words. But I have sold novels as short as 40,000 words for major advances. And there are many great works of literature--like THE STRANGER--that fall far short of 50,000 words, and yet are still considered novels. There is no absolute word count or line in the sand that demarcates a novel from a novella.
Typically, if a writer asks me how long a novel must be (minimum), I'd say to aim for at 60,000 words, though at least 70,000 would be idea, and if it's even longer, that's great--though if it exceeds 100,000 words then I might start to worry (unless it's in the hands of a master).
However, do keep in mind--and this is the important point I want to make here--that with the ebook revolution, many self published authors are discovering that many readers are just fine with reading shorter novels, especially if they are priced at $2.99 or less. Thus if an author of commercial fiction came to me with a 100,000 word manuscript that could easily and naturally be divided into two books of 50,000 words each, and he asked me if 50k words was too short for each novel, I would say no, and in most cases suggest he split them into two. For me to tell him not to do it, and that 50k words is just a novella, would be bad advice.