How long should a book be? Chuck Sambuchino, in his useful blog Guide to Literary Agents, argues that while a brilliant book can be any length, it makes sense to aim for a word count which appeals to agents. For adult novels, both commercial and literary, this apparently means between 80,000 and 100,000 words, with 90,000 words looking the most appealing.
He gives analysis of his thinking on his blog, with specific suggestions for crime novels, sci-fi and fantasy and young adult books. Worth checking out.