How do you know if your story should be a short (my definition is around 4,000 words. More on that later), a novella(20, 000 to 60,000 words, approximately), or a novel? (For those of you who don't write, a mass market paperback is usually 80,000 to 100,00 words. )
This question comes up a lot on my writer's lists. Once you've written a number of stories, I think you get a feel for the word count you're going to end up with, but it isn't always clear. Some seasoned writers intend to write a short and get so caught up in the characters that they realize there's a novel in there somewhere. But where experience helps is when a seasoned writer can keep that story short and then go on (maybe quite a while later) and extend those characters into a novel.
The word count is influenced by many things, but I'd say time frame of the plot and size of the idea are most important. Many good short stories show a span of time, but the strongest ones seem to center on a specific moment in time. They also focus on a central idea and explore it in brief depth. The bigger the idea, the more words you need to explore it. The more complicated the plot, the more words you need. The more shifts in time you have, generally the higher the word count.
If you read a call for submissions to an anthology, check the word count. If it's literary erotica, you're going to see an upper limit of around 5,000 words. That means you have to cut to the chase, hit the ground running... whatever term you use for getting down to business quickly. Within the first paragraph, you should have broached the topic of your literary thesis (the main idea) and made some statement about who your main character is. Keep this mantra in mind - "What is this story about?" Answer that question, and only that question. Do it succinctly.
If you're writing genre erotica (romantic erotica) and you're dealing with an e-publisher, the definition of a short story ranges by publisher from an average of 4,000 words to a minimum of 10,000. Read the call for submissions carefully. A longer word count gives you more room to play with time, theme, and characters. You can (although I wouldn't advise it) take much longer to define your central character and pose your problem.
Literary erotica publishers rarely take novellas. If you can't trim your story under 7,000 words, and the upper limit on the call is 5,000, write a respectful letter to the editor asking for permission to submit it anyway. The worst that will happen is that you'll be told no. Trust me - editors do not come to your house and make you write "I will only try to submit stories that fall within the word count guidance in the call for submissions" five hundred times in an essay book.
Many E-publishers of romantic erotica will accept novellas. But do me a favor. At the end, don't do the wrap up paragraph. Resist the temptation to write "Mary Ann went on to become a successful fashion designer, and married Jack, while her archenemy from design school - the scheming bitch who tried to steal Jack away from her - became the night shift fry cook at a skid row diner. " I should be able to imagine that continuation from what you wrote in your story. Even though it's a novella, it is a self-contained story that ends with the last period. Anythiing that happens after the story you're telling has ended belongs in a different book.
Novels, as with this blog entry, can start as a simple enough idea, but there are so many complications that it takes a while to work through to the end. You'll know if you don't have a big enough idea to sustain a novel if you get bored and run out of steam at about the seventh chapter. The fun of creating your world is past. Your characters are defined. Your problem is stated and you've set about resolving it. And then... it seems as if you have to throw in ridiculous complications to keep it going. If you're tempted to take the shortcut and zoom on to the end when you're only 20,000 words in, do it. Then find a publisher who takes novellas.
But what if you're 200,000 words into what was once a short and you're still going strong? Trilogy, honey. Start thinking trilogy.