All credit to Lee Masterson! Clipped from this handy website:
Recently we received several emails asking the question: "How long should my story be?"
The simple answer is: As long as it takes to tell the whole story.
However, there are certain word lengths that most editors prefer to see when submitting work.
Here is an approximate guideline for story lengths.
up to 100 words
This very abbreviated story is often difficult to write, and even harder to write well, but the markets for micro fiction are becoming increasingly popular in recent times. Publishers love them, as they take up almost no room and don't cost them their budgets. Pay rates are often low, but for so few words, the rate per word averages quite high.
100 - 1,000 words
This is the type of short-short story you would expect to find in a glossy magazine, often used to fill one page of quick romance (or quick humor, in men's mags) Very popular, quick and easy to write, and easier to sell!
1,000 - 7,500 words
The 'regular' short story, usually found in periodicals or anthology collections. Most 'genre' zines will features works at this length.
7,500 - 20,000 words
Often a novellette-length work is difficult to sell to a publisher. It is considered too long for most publishers to insert comfortably into a magazine, yet too short for a novel. Generally, authors will piece together three or four novellette-length works into a compilation novel.
20,000 - 50,000 words
Although most print publishers will balk at printing a novel this short, this is almost perfect for the electronic publishing market length. The online audience doesn't always have the time or the patience to sit through a 100,000 word novel. Alternatively, this is an acceptable length for a short work of non-fiction.
Most print publishers prefer a minimum word count of around 70,000 words for a first novel, and some even hesitate for any work shorter than 80,000. Yet any piece of fiction climbing over the 110,000 word mark also tends to give editors some pause. They need to be sure they can produce a product that won't over-extend their budget, but still be enticing enough to readers to be saleable. Imagine paying good money for a book less than a quarter-inch thick?
Epics and Sequels
Over 110,000 words
If your story extends too far over the 110,000 mark, perhaps consider where you could either condense the story to only include relevant details, or lengthen it to span out into a sequel, or perhaps even a trilogy. (Unless, of course, you're Stephen King - then it doesn't matter what length your manuscript is - a publisher is a little more lenient with an established author who has a well-established readership)
In most cases, industry standard preferred length is 250 words per page... so a 400 page novel would be at about 100,000 words. If you want to see what size book is selling in your genre, take a look on the shelves. If the average length is 300 pages, you're looking at a 75,000 word manuscript (approximately)
One reason it's harder for a new author to sell a 140,000 word manuscript is the size of the book. A 500+ page book is going to take up the space of almost two, 300 page books on the shelves. It's also going to cost more for the publishers to produce, so unless the author is well known, the book stores aren't going to stock that many copies of the 'door-stopper' novel as compared to the thinner novel.
Remember, these word- and page-counts are only estimated guides. Use your own common sense, and, where possible, check the guidelines of the publication you intend to submit your work to. Most publishers accepting shorter works will post their maximum preferred lengths, and novels are generally considered on the strength of the story itself, not on how many words you have squeezed into each chapter.