Two years ago, I finished a draft for a novel and promptly put it on the shelf. I didn’t shelve it because I thought it was bad—quite the contrary, I think it has a lot of potential. I shelved it because I thought it was too long to have a decent shot at publication as a first novel. The draft came in 241K words, and with revisions, I expect the final length to be in the ballpark of 160K words. That exceeds what is generally considered “marketable” for a debut novel. So how long should a novel be?
This is actually fairly popular topic of discussion, and there are not many clear-cut guidelines. Numbers often vary widely, but many speculative fiction publishers put an upper word limit of 120K words on unsolicited novel manuscripts. Various agents and other writers suggest that debut novels not exceed 100K. Of course there are exceptions (aren’t there always), and the upper limit often varies by genre. Colleen Lindsay, who works with the Penguin Group and is the manager of Book Country, an online writing community, has a wonderful post on approximate word counts for various genres of novels. For science fiction novels, 80-100K seems to be the sweet spot. This bodes well for the novel draft I finished last December, but is likely to keep my older and longer novel on the shelf a little longer.
D. Thomas Minton 
Sounds like the longer novel will have to wait a while, but that can be a good thing. The illusion among readers will be that you got right to work on your comparatively epic second novel hot on the heels of completing your shorter fist novel. Throw an excellent short-story collection into the mix for your third book, and everyone will wonder where you find the time to get it all done!
I’ve contemplated splitting the longer into a multi-volume story, but the older I get, the more I hate multi-volume stories not to mention the short stroy writer in me cringes). Alternatively, I may extract one of the story arcs for another novel and edit the remaining bits down to ~100K, but that’s all for the future….
A work should be as long as it should be. Write how long or how short you want it to (or how long the text justifies it to be). Let your agent worry about the rest 🙂
I agree—long enough to tell your story and not a word more! Sure a knock-your-socks off novel (or short story for that matter) will sell at any word count, but I also believe in giving myself the best shot I can get. Once a writer has shown an established sales record, longer novels are less risky from the economic perspective of a publisher.