So mostly for fun I’m taking a writing class. The professor told us last week that we should make a habit of writing 1000 words a day. In 90 days we will have a full novel. (For the sake of the question, let us ignore whether or not your story can be told in 90,000 words. The professor did address this argument.)
This made me curious though. Is it true that the average novel is approximate 90,000 words long?
If I remember correctly the average novel is between 75,000 and 100,000 words. Several books on writing, as well as trade magazines, suggest that it is typical for the first draft to be on the slim side and to pick up more words in the first rewrite (filling in logic gaps mostly). Later on there may need to be edits to cut out extraneous parts as the novel presses into the higher, less publishable lengths.
You’d be suprised how much actually happens in a novel, and how easy it is to fill up space with things not happening if you write a set amount a day without a plan of action. Most stories can be told in 90,000 words if you remember that interesting asides and showing off your creativity/intellect/knowledge do not constitute part of the story.
Good luck and have fun.
Too many.
Write your novel, as your professor says, then edit it for brevity.
Cut it by 30% to 50%.
It will tighten your work, & improve the style.
You may wind up with a novella, but it will be more publishable.
Ask him about what I said, see if he agrees. 
Novellas are the single hardest type of fiction to publish. It is almost impossible to get one into print unless you are a very big name.
Novels vary a lot in length, something that’s easy to check by walking into a bookstore and comparing thicknesses. The average length of a novel also varies greatly among different genres of fiction and over cycles of time.
90,000 words would be impossibly short for a fantasy novel these days. Most run two or three times that. People who read fantasies like to get lost in worlds and not come out for as long a time as possible.
Bestsellers work the same way. People want heft for their money, and get it. Almost all bestselling fiction is 400 pages, which is a good 150,000 words.
Even mainstream fiction tends to be heftier than 90,000 words. I checked some new books on my to-be-read shelf and even the smallest seemed to be at least 120,000 words.
The flip side of this is that first novels are often shorter, as few first novelists can handle the plotting and other demands of a long novel. Editors will want them edited down to as tight a length as possible. So 90,000 may be a good starter length.
I’m not sure that anyone can apply “average” to a field as broad as novels. Most probably, your professor is giving you the minimum possible goal to make it seem achievable.
On the other hand romance novels, which, while not particularly revered as literature remain a constantly growing market, can be as small as I think 60,000 words. And many excellent mystery novels I’ve read are almost as short.
My advice would be to first, write the story you want to tell, then, like Bosda said edit it down by 20-33%. After that is time to see where you can market it. If you have a market in mind, consider what publishers would be good for that market, and check their own guidlines. IIRC Writer’s Market and many websites offer guidelines for manuscript length.
There are always exceptions, but the vast majority of major publishers in romance or mystery do not buy 60,000 word novels. Some electronic publishers do, however, and so do some small presses. YA fiction is also usually around that length. I should have also noted that while print novellas are a difficult market, electronic presses are beginning to offer more novellas, a sign of the number of people who don’t like reading long works electronically. I still think in print, which still dominates in numbers.
I checked the Romance Writers Report, the magazine of the Romance Writers of America, for their Comprehensive Market Report. Not every publisher gave a word length, but here are the ones who did:
Avalon: 40-60,000
Avon: 100,000
Ballantine/Ivy: 100,000
Berkely/Jove: 100,000
BET Books/Arabesque: 85-100,000
Dorchester: 90-100,000
Ellora’s Cave (electronic): 45,000+
Genesis Press: 80-100,000
Harlequin Next: 75-80,000
Kensington Arabesque: 80-100,000; Regencies: 75,000; Brava: 100,000
Mills & Boon Historical: 75-90,000
Pocket Books: 90-100,000
Red Sage single title line: 55-75,000
St. Martin’s: 100-120,000
Tor/Forge: 80-130,000
Tyndale House: 75-100,000
You can see a range here, with historicals usually being longer than other types, but 100,000 is the new normal.
Which ones? Like most of the other contributors to this thread, I feel that most writing needs to be cut rather than stretched. In my own work, I inevitably shorten when I edit, pushing for clarity and brevity. The few places where I add are overwhelmed by the many where I cut.
(I just cut a whole paragraph out of this post, and it’s better for it!)
Depends on the genre. The book on writing Fiction proposals by Elizabeth Lyons has a table of lengths per genre. First books should be on the short side, later books can be longer. The sf entry was 65,000 - 90,000, for a first book.
I spoke to an SF small press publisher at a writers conference. He said that he didn’t take books over 130,000 words, since they were just too expensive to publish.
I’m particulary sensitive to this, since my book was 200,000 words. Based on this input, I finally realized that it was actually 2 books, about 70,000 and 130,000 words respectively.
BTW, the agents at the agent panel said that they didn’t take short stories or novellas. The only significant paying markets are mystery and sf, and those editors preferred to work directly with authors.