When you're starting off in your writing career, it's very easy to go around saying that you're writing the great American novel (a myth that I plan to debunk in a later post). You get a great idea for a book and you plunge into it will all the steam you have. The problem with this is that you're not yet ready to tackle the prospect of novel writing. Even if you get your book through a few drafts, you have the daunting situation of being an unknown author trying to pitch your work to an agent with no credits to your name. While agents are in the business of looking for the next big writer, it's still a good thing to have some writing credits to your name so that prospective agents see that you have spent time getting projects finished from initial thought to final print.
It comes down to numbers and word count. Novels are at least 40,000 words. Novellas run between 17,500 and 40,000 words. Novelettes are between 7,500 and 17,500 words. Short stories are 7,500 words and under, with flash fiction topping off at 500 words. For the sake of argument and to spare me from strenuous arithmetic (I'm waiting for my mug of coffee to kick in), let's say that I write a hundred words a minute. At that rate, a novel can take as little as six or seven hours and a short story can be done in about an hour and a half. Thus I can get through a short story draft sooner, get to the revisions more quickly, edit and rewrite. In the end, a short story can be ready in about a month, whereas a novel takes years to complete. Once the short story is done, it can go out sooner to magazines and you get that acceptance or rejection notice accordingly earlier. The point that I'm trying to make (for those of you merely skimming this post) is that if your work is really good you can sell perhaps half a dozen short stories in a year, building up credibility as a writer so you can approach that agent with your masterpiece.
That works for the business of writing, but there's also a craft issue. Writing novels takes the pressure of the word count off of your shoulders. You don't have to be economic with your word choices or the frequency of your metaphors or the length of your descriptions. With shorter fiction, on the other hand, you still have to write a narrative from start to finish, but with that ceiling overhead. As you get better at short stories, you learn how to identify stronger aspects of the story and give them attention and development. This will serve you later on as a novelist because as much as a limitless word count can be a blessing, for the reader it can be a burden. Just think. All those really big books with small print and no pictures! What if it's boring? Novelists who hone their skills through shorter fiction, I believe, are less likely to bore their readers. The more you practice with shorter and shorter fiction, the better you'll be. I have a lot of respect for people who can master flash fiction; I haven't had the guts to try that yet.