While talking about writing, it makes sense that sooner or later, we'll touch upon the subject of word counts. Currently, there are about five different categories for story lengths that I'm going to go over really quick for those of you who are unaware or need a reminder/refresher.
(An example of Word Count in MS Office, and a teaser treat for those of you paying attention.)
Flash Fiction - This is a little hard to pin down, as it largely depends on what the publisher is looking for. Some magazines will accept works as large as 1,000 words as flash fiction, while others require fewer than 300 or even 100 words. Of course, this also includes the trend of 2-sentence stories, which could be as few as ten words and still manage to convey a story. The point is, these are stories which can be read in a matter of minutes by the average person; little bits that allow people to break away for a moment even if they don't have much time.
Short Story - The next step up, these start where the Flash Fiction ends, right around 1000 words and go up to about 7,500 words. Most people are pretty familiar with short stories. These are the ones you generally find populating anthologies, or in magazines. With the current self-publishing movement, you can also find a supply of short stories standing on their own, either for free or at the $0.99 mark. Also, before I get much further, I'm going by the word counts listed under the category requirements for the Science Fiction and Fantasy Writers of America's Nebula awards. While there will almost always be some wiggle room where word counts are concerned, the counts they suggest are fairly established and give what I think are good averages.
Novelette - Now we're getting up there. Ranging from 7,500 words on up to 17,500. Now we're getting into some meaty stuff. For an average reader, this will be a couple hours worth of entertainment. Something to read on a long bus ride or waiting at the DMV. Like the short stories, it's not uncommon to find some of these standing on their own on Amazon, but you're also likely to find a few stories of this length mixed in among the short stories in a collection.
Novella - This is finally starting to get up there. At 17,501 - 40,000 words, these are the short 'books'. Still, these are generally good for an evening or even a weekend read depending on how fast a reader you are. You can also find these as a kind of main course in a good-sized collection, where several short stories lead up to or even tie in to the final novella story.
Novel - This is a book. 40,001 words is the minimum required for the Novel category in the Nebula awards, but I do take exception to this just a bit. Most publishers won't consider a manuscript book worthy unless it is at least 50,000 words, and some require even more depending on genre. Still, I think everyone here knows what a real book requires, so not much more really needs to be said.
(See the difference in thickness?)
So now the question is, how long is your story going to be? Well, that all depends on you and your story. When it comes to you, it's a matter of writing style. Is your prose full of description and philosophical musings? Or do you tend to get straight to the point, forgoing needless details like what your character had for breakfast or what the particular shade of blue the sky is reminds him of? Of course, writing style is only going to carry you so far. It might turn a short story into a novelette, but beyond that you're probably cramming extra stuff in there to force a word count, and that is almost never a good idea.
What really dictates a story's length is what is in the story itself. How many characters are included? How many hoops are there for them to jump through before the climax? What about side-plots? Are all loose ends tied up or are they left out there for possible future use?
I think most of these things will be dictated by the story itself and if you listen, you'll know what to put where, and how to draw the lines. One thing I would suggest an author never does though, is to force a specific word count. Many times, forcing a book to stretch fills it with unnecessary details that can spoil the pace, as well as introduce details that don't make sense with the rest of the story.
A perfect example of this, for me, is Rick Hautala's The Wildman. Some may enjoy it, but to me almost the entire book felt like it was a novelette stretched out to novel length for no real reason. The second to last chapter can be summarized in three lines: "I can't go on", "I wonder if I'm dead", "My body is still moving somehow." rinse, and repeat for at least fifteen to twenty pages. Seriously. It's almost literally those three lines, drawn out into paragraphs, and then repeated over, and over, and over. If that isn't forcing a word count, I'm not sure what would qualify.
Yes, it can be frustrating when you're writing and your story suddenly seems to fall well short of what you thought it would be, but rather than trying to stretch things out, read it over, see if there's anything you forgot to add in. If so, great! If not, don't force it. More often than not, when you use force to try and make something do what you want and not what it's supposed to do, you're going to break it. There is plenty of room out there right now for stories of all lengths. Don't break your hard work trying to force it to be something it just can't be.