However, it's a question only first-time authors ask, because of course the real answer is "as long as it takes to tell the story and no more". You should read the full thing (see the link above), but the crux of her answer is: around 100,000 words for adult fiction.
Now that figure of 80K-120K gets bandied about a lot. But it ignores the fact that the person asking the question was that undiscovered first-time author. I'd suggest that Zigner's sums are wrong for two reasons, at least in the UK.
First, very few new writers get contracts with one of the big five publishers. Instead they're picked up by smaller publishers or even publishing collectives such as ourselves. For small publishers, books either have to be bought by wholesalers in bulk, or – much more likely – they're stocked and sold by booksellers in bits and pieces. Small publishers then have to send out books by post individually.
The point is that, in the UK, anything under 25mm thickness can be sent as a "large letter" for around £1.60. Anything thicker than that is classed as "small parcel" and costs around £2.90. That extra £1.30 comes straight out of the profit margin for author and publisher. Typically, a bookseller will expect £4.00 profit margin ("discount") from a £9.99 book. If it costs £2.90 to send the book on top of that, the publisher is left with just £3.00 to produce the book – not very much. The "large letter" option is much better as it leaves £4.30 to play with. Very roughly, a 300-page paperback is around 25mm thick. In my experience, printed in something like Minion or Garamond at 11pt, that's approximately 75K-80K words.
The second factor is easier to understand. If a reader is new to an author, then they're more likely to read a short example of their work first – it's common sense. Here at Comely Bank Publishing we call these "breakthrough titles", and at the foot of the blog you'll see a raft of examples of short novels which are likely to be the first books by those authors that you've read. I know that Zigner makes exceptions for literary fiction, but I think it applies to all adult fiction really. Once you've read one of the author's works, there's every chance you might go back and read a longer one, but then you're in a different league altogether.