Here's a detailed article about novel lengths
and why they are the way they are.
The FIRST consideration should be story need. Write the length that the plot requires. A novel can be anywhere from 40,000 words to well above 120,000 these days.
Publisher demands have depended partly on popular format, so as prices went up, people made fatter books. The problem is, most of that is bloat; it degrades the product quality. I hate
bloated novels. A fat book makes me suspicious unless I know the author is good for it. If you give someone a target in advance, though, at least they know where to aim.
But the worst problem is when people take a story that's already written and cut it down without regard to its infrastructure. This is butchery,
and the result is often illegible. You can write a book and subdivide it into serial chapters if you know what you're doing. But to take, say, 800 pages and divide that into thirds when it wasn't meant to be? Chances are it will suck mightily.
Never, ever do anything which degrades the base product quality. Movies are having a big problem with this currently where the fulminating mess that is copyright has interfered with the ability to tell a good story. Fuck you, my money can stay in my pocket. People are reading fewer books today. Part of that is changing culture, but part may be because of really stupid publisher decisions like these.
On the bright side, electronic publication removes the size constraints. You can write and publish whatever size your story needs to be. Your readers can buy whatever size they want. This is an advantage that paper will never be able to compete with, and it's bringing back gutter-length fiction: stuff in the 10-30,000 range that's all but impossible to sell in print. I'm not into ebooks but I am very happy with the impact on size diversity. There's much less temptation to hack or bloat in an electronic environment.