Word Count for first time novelist:
Adult fiction: 80,000-100,000 words.
Young Adult fiction: 40,000-60,000 words
Look, publishers want a product they can sell to the reading public for $24.95 a pop. How many adult novels are you willing to fork over $24.95 hardcover if they're only 40,000 words in length? Not many. Major publishers want 80K-100K novels because they're easier to market and sell to the reading public—a reading public who already has certain expectations about how long books should be and what they're willing to pay.
Now, just when you thought it was safe to write an 80,000-word novel, you should know that we've heard agents give some genres of fiction a bit of flexibility with length:
Erotica novellas/short story collections: 40,000-60,000 words
Cozy mysteries only: 50,000-70,000 words
Most romance novels: 50,000-70,000 words
Short Story Collections: 40,000-75,000 words
Historical Fiction: 80,000-140,000 words
Adult Fantasy: 90,000-140,000 words
(FYI: a 140,000 word debut novel is about as scary as Joan Rivers' face. So if you're shopping a 140K chubby moon-glop monster, you might want to consider NOT advertising this fact in your query letter).