Summarizing what agent Colleen Lindsay, blogging at
http://theswivet.blogspot.com/2008/03/on-word-counts-and-novel-length.html, says about word counts: It’s all about the genre, often the subgenre.
YA Fiction: 50,000 - 80,000 words, rarely 90,000
Urban Fantasy: 80,000 - 90,000
Paranormal Romance: 80,000 - 90,000
Mysteries and Crime Fiction: Cozies 60,000 - 70,000; all others 80,000 - 100,000
Science fiction and Fantasy: 100,000; Epic Fantasy 120,000 - 130,000
Mainstream fiction depends on subgenre.Chick Lit 60,000 - 80,000; Literary Fiction up to 120,000 but lately trends to much shorter; Thrillers 90,000 - 100,000; Historical Fiction to 140,000.
Anything under 50,000 words is usually considered a novella, which isn't something agents or editors ever want to see.