One word: Economy.
And a keen understanding of characterization, setting and scene, conflict...
That's more than one word. Actually, that's a lot of words.
But they apply to more than just flash fiction.
The length of any story can be very accurately predicted based upon these elements. Let me explain.
Economy: An economical writer (IMO the most enjoyable type of writer to read) doesn't waste words, doesn't repeat what's already been said, chooses the 'less is more' path to revealing information to the reader.
As a very simplistic example, one of the most familiar sentences in the English language is, "The quick brown fox jumps over the lazy dog." In its context, this sentence has purpose, and every word is necessary, because the purpose of this sentence is to use every letter in the English alphabet at least once. But in terms of story, this sentence is far too long, with 1/3 its words completely unneccessary. We can get the gist of the sentence perfectly well with "The fox jumps over the dog." If we want to add to characterization we might say "The fox jumps over the lazy dog."
But the point is, a writer who is capable of managing his use of the words within the language can come to an accurate self-measure of story length before he's written the first word.
And he does this by carefully disciplined management of characters/characterization, scene and setting, and conflict.
It's almost a mathematical formula:
#CH X SET + #SC X CON = #WORDS
That's a purely arbitrary formula. If I ever figure out an exact one I'll let you know, but don't hold your breath. I'm no mathematician.
But let's put it into the context of story construction:
Number and Complexity of Characters: Characters are 'people' (even if they're not human) who by their very existence deserve acknowledgment. Think of them in terms of yourself. You want people to know your name, and you want people to know some information about you--especially information that is relevant to current circumstances. For example, you won't talk about your childhood memories at a job interview. You WILL talk about High School memories at a High School reunion.
So, when you introduce a character, you want your reader to know her name and to know something about her that is relevant to the story. For example, the first sentence of Stefanie Freele's delightful "James Brown Is Alive and Doing Laundry in South Lake Tahoe" (Flash Fiction Online January, 2008) reads:
Stu is driving to South Lake Tahoe to take his post-partum-strained woman to the snow, to take his nine-week-old infant through a storm, to take his neglected dog on a five hour car ride, and to take himself into his woman’s good graces.
In one sentence we know everything we need to know about Stu in order to let the story progress. We know he's married, he has a child, he is experiencing some strong emotions concerning both of them, we know he's contemporary with us. (We also know the source of conflict, and the setting, but we'll discuss that later.) All in one sentence and using a stark economy of carefully and brilliantly chosen words--a few words to express worlds of meaning.
So one point to make in discussing characterization and story length is the complexity of the character.
A simple character is one who needs little description. (I had a clear and vivid image in my mind of what Stu might look like after only one sentence.) A simple characters is one whose conflict can be resolved within the context of a very small arena of his life. (I don't need to know Stu's entire life history to make the story work. All I need to know is that he's dealing with a post-partum wife the best he knows how. That's enough to endear the reader to him.)
A complex character may have his entire life story told within the framework of the story. A complex character has traits--either physical or emotional or concerning his character, or a combination of these--that are integral to the conflict and the resolution thereof. A complex character will make profound discoveries and changes during the course of the story.
A simple character needs very few words to 'flesh out,' to make REAL to your readers; but a complex character will require numerous paragraphs, chapters even, to become a whole person to the reader.
Obviously, a simple character isn't going to fill a novel; a complex character isn't going to fit in a flash fiction story.
The second aspect of character that affects story length is simply number. The number of characters. Every named, acting character takes space in a story. Each one deserves the same amount of attention any other character deserves. If you want your reader to feel deeply for a character (love or hate or somewhere in between) that character needs time (translation: words) in your story.
A novel can support a broad range of named, acting characters, and can support shifts in POV between several characters. At the other end of the spectrum, a flash fiction story can only support 1 to 3 named, acting characters and generally only 1 POV character. In the context of the novel, a single scene can't very successfully support more than a few characters, and generally a POV shift signals the end of a scene. I guess that's a plug for the study of flash fiction. ;-)
Setting and Scene: Setting is where a story takes place; a scene is an interval of time in which a specific action takes place within a particular setting. Both have an impact on story length.
It comes down again to complexity and number, just as with characterization--managing the complexity of the setting and the number of settings and scenes within those settings.
As an example, from "Just Before Recess" by James Van Pelt (Flash Fiction Online March, 2008):
Parker kept a sun in his desk. He fed it gravel and twigs, and once his gum when it lost its flavor. The warm varnished desktop felt good against his forearms, and the desk’s toasty metal bottom kept the chill off his legs.
Today Mr. Earl was grading papers at the front of the class, every once in a while glancing up at the 3rd graders to make sure none of them were talking or passing notes or looking out the window.
Four sentences this time. The setting is a third grade classroom. You can see it in your head. I know you can. It's an image burned into the collective Western brain, even if you've never stepped inside the doors of a school. We've seen this place in countless films and television shows. It takes very little physical description to put the reader into the middle of this story. We don't need to see the maps on the walls or the pull-down blinds or the industrial berber carpet or the shelves of books and cubbies to know they're there. That's a simple setting, because it's a familiar one.
The less familiar a setting is to the collective human experience, the more description it requires.
You can do the math yourself by now.
The more complex a setting, the more description it will require. So, complex settings are not suitable for flash fiction. But are complex settings required for novels?
No. Because a novel has scenes to work with.
A story is a series of 'this happeneds.' It also just so happens that the 'this happened's occur in 'this places.' James Van Pelt's story is something that happens in one place. One setting. And with one 'this happened.' One scene. Very Short Story.
A novel can occur in any number of settings, with any number of scenes in which 'this happeneds' take place. A novel may have one setting, but numerous scenes, or numerous settings AND scenes. I don't know of the existence of a novel that relies on one scene. Correct me if I'm wrong. Is there such a thing out there? I hope not. Who would want to read it? I swoon at the thought. At any rate, management of the complexity of settings and the NUMBER of settings and scenes will steer the length of the story as well.
Which leads right into...
Conflict: Your setting creates a stage on which the action takes place. Your scenes are frameworks in which that action occurs. Your conflict happens within your setting and is resolved within a series of scenes.
What is conflict? In a nutshell, conflict is the impetus for action. It is the thing that causes your main character to want to do something to change what's wrong with his world (resolution).
As an example, from Patrick Lundrigan's "How High the Moon" (Flash Fiction Online, September, 2009):
“You’re a robot, you know. I made you.”
“I’ve heard that before,” Nomie said. She put the tea tray down and settled into the lawn chair. “But I don’t think I’m a robot.”
“Programming,” Manny said, “I programmed you not to know.” He blew on his tea and sipped. Just the right amount of sugar and cinnamon.
“Dear, I have news for you. You’re the robot. I made you.”
In a few short lines we are exposed to the conflict of the story--a couple's argument about exactly who is the robot in the family. It's a simple and subtle and sweet conflict--at least you'll discover it to be so when you read the story.
Will it take much to resolve the conflict?
It could, I suppose. But the simplicity of the setting and the characters combine with this simple conflict to hint to the reader that the resolution will be a fairly simple one as well.
None of these require a great number of words to accomplish. This is a nicely done story within the framework of 1000 words or less.
So how do we make the conflict more complex in order to flesh it out to novel length? The answer is secondary conflicts. Let's say the conflict over who is actually the robot evolves, in the context of the argument, into a discussion of the factory where she/he was made, or the man who invented them? Perhaps the truth of the matter is paramount to a matter of national security? As conflicts breed other conflicts, the space required to a) explain them and b) resolve them all increases exponentially.
A disciplined writer of short fiction will carefully rein in the tendency of the creative mind to include additional problems that will lengthen the story. In "How High the Moon," Mr. Lundrigan shows remarkable restraint in avoiding any secondary conflicts and focusing the conflict on the relationship between this man and his wife to make a lovely story that is less about robots than it is about love and loyalty.
On the converse, a novelist can let the imagination run wild, restraining himself only with the knowledge that every conflict he introduces MUST be resolved. Even novelists must exercise a certain amount of restraint, otherwise he'll be writing the novel that never ends.
In conclusion, it's a combination of the above that determine story length. The longer the story the less one must exercise discipline.
That's the beauty of practicing the craft of very short fiction, however. All writers could benefit from the exercise of discipline, no matter what length fiction they write.
Flash Fiction. Try it. You may not like it, but you just might learn something from it.