Remember when you're girlfriend told you size didn't matter? She was lying.
If you want to sell your book, particularly your first book, size does matter. If your book is too short or too long, agents and editors will see it as unmarketable. Not sure how much truth there is to their assumptions. Some of my favorite books are way over 500 pages while others are well under 200. You may ask, "How many words was Catcher in the Rye or The Fountainhead? Those didn't fall between the word count boundaries." And your answer is, when you're J.D. Salinger or Ayn Rand, you can write whatever you want no matter how short or how long. But you're not Salinger and you're not Rand.
But even the boundaries seem to be a bit disputed. Some writers or publishers say a novel should be between 80k and 90k words. Others say they won't accept anything under 100k. One non-fiction writer on Backspace says his contract specifies between 50k and 65k. Another memoirist says his contract specifies 70k. Like everything else in the publishing world, there are no hard and fast rules. To figure out how long your novel should be, look at other books in your genre and look at the publishers. Harlequin romances, I believe, are around 65K while Kensington thrillers are way over 100k. The average P.I. novel seems to be around 80k while thrillers fall around 90k. Literary fiction is all over the place Your best bet is to stay in the middle, aim for an average length as it pertains to your genre.
Let me preface this by saying, I went to art school. Don't hold it against me. I was taught to write where the story takes me and write until the story resolves itself without worrying about plot and manuscript length. I did that for Street Walk and trust me, it doesn't work. My first draft came out to be 67,000 words, way too short for a novel, way too long for a novella. But the second draft was a little longer, and the third even longer, until the sixth draft was of acceptable length (although still a little on the short side). But with each revision, I wasn't just fluffing it up; I was adding description, slowing down action scenes, telling the story so the readers can see it. All that takes words, words I needed to add.
On the flip side, I've never written anything that was too long. Don't know if it's my ADD, laziness, or that I'm just careful with my words. But if you're the type of person that writes too long, try this: go through the book, one page at a time, and omit needless words. Ask yourself at every sentence, what this sentence, what one word, is doing for the overall story? If you don't have an answer, the answer is cut it. Say your hero gets shot. Do we need to see him at the hospital? Is that furthering the overall story? Or could you just summarize it in one line, "After he was discharged..."
The other, and better option, it to write your novel within the proper word count the first time around. Don't worry, it's almost as simple as it sounds. Planning ahead is the first step. I've gone on record time and time again saying how averse I am to outlining. Well, I've been converted. Outlining before you write helps you write better first drafts and therefore, saves time. When you write your outline, make a note of what word count you should have at each turning point. It will help you stay on track and know if you're giving too much or too little description. If you reach the midpoint and you're only at 35,000 words, chances are, you wrote too fast, skimming over important details. If that's not the case, you may need to add another plot line or further develop a secondary character.
Unfortunately, we live in a world where size does matter. And while there are hundreds of exceptions to the word count rules, it's much easier to sell a book if you're not one of them. Save yourself time and energy. Plan ahead. Think about word count as you write instead of waiting until the end (and then having to write five more drafts to get it right!)