According to the Science Fiction Writers’ Association (SFWA), the difference between a novel and novella is purely whether the story has forty thousand words. At that point or more, it’s a novel: fewer, and it’s a novella.
Unfortunately a lot of people seem to think of novellas as a lesser art form. I think that’s unfortunate. I find it harder to write good novellas than good novels. The novel format gives the author room to breathe and let ideas flourish organically, which can be lovely, but the novella is its opposite: every page must grab the reader, every sentence must crackle. The brevity of the format necessitates immediacy in style.
Besides, many of the things you think are novels really aren’t. C.S. Lewis’ magnificent Narnia books are almost all novellas (The Lion, the Witch, and the Wardrobe comes in at thirty-six kilowords, for instance). John Steinbeck’s Of Mice and Men is composed of thirty thousand perfect words. George Orwell’s Animal Farm, Charles Dickens’ A Christmas Carol, and Ernest Hemingway’s The Old Man and the Sea are all novellas, as is Stephen King’s masterpiece Rita Hayworth and the Shawshank Redemption.
The question really shouldn’t be, “is this story long enough to be taken seriously?” It should only be, “is this story worth the money I spent to buy it and the time I spent to read it?”
And that, dear friends, is a great question to ask of anything from flash fiction to Gone With the Wind. (Over four hundred thousand words, if you’re counting.)