Troilee said:
The length of a novel is entirely dependent on the genre. A full length YA is about 50k-60k words, a category romance is around 60k-70k, spec fic stuff tends to be in the 80k-100k range and historicals can go as high as 120k, depending on the subject matter. Of course, you'll always have big authors who have about a million words in their books, but they're the exceptions.
No, you're confusing guideline requests with what is and what isn't a novel. They have nothing to do with it. All those numbers do is tell you what length given publishers ask for, and what the average length is. They have zero to do with what is and what isn't a novel. A novel remains a novel, whatever its length: 30K is always a novel, and so is 40K. So is 300K or 400K. Whether or not an individual publisher wants novels of this length is a different matter, but some very good novels have been published in the 30,40, and 50K lengths in nearly every genre.
But it was the genres of science fiction and fantasy that set the 40K length, and they did so for good reason. They did it because of serialization. These length limites make serializing novellas and novels very easy, and allow a novel at the 40K length to be published later on. Not too many years ago, published novels of 40K were pretty common.
And in some areas, they still aren't terribly uncommon. The Bridges of Madison County was only 35K, and it stayed on the bestseller list forever. The Notebook was 49K, and it was a huge bestseller.
In adult fiction, and in children's fiction to a lesser degree, and isn't a novel is determined by writing technique.
There is no such thing as a "full-length" novel. Something is either novel or it isn't, whether it's 30K or 300K. When's the last time you heard anyone use the term "partial length" novel.
And spec fic generally runs up to 120K, and historicals up to 150K, but every length is a novel. You do not write a novel the same way you write a short story. A 30K short story is going to be overlong, padded, and boring, just as a 5K novel would be cramped and unreadable.
A novel is a novel because it's written using novel techniques, not because this publisher or that one only wants novels of certain lengths.