If you've ever wondered how many words are appropriate for your manuscript, you should check out this article: Word Count for Novels and Children's Books - The Definitive Post by Chuck Sambuchino.
As an editor at Writer's Digest, Sambuchino edits the Guide to Literary Agents and the Screenwriters and Playwriter's Market. In his article, he has broken down the general rules for word counts by genre. While he notes there are some exceptions to these rules, he also notes that new writers cannot assume they are the exception to the rule.
Here's a synopsis of the counts; for details and further explanations, read his excellent post:
* Adult Novels: Commercial and Literary - 80,000 and 89,999
* Sci-fi and Fantasy - 100,000 - 115,000
* Middle grade - 20,000 - 45,000
* Young Adult - 55,000 - 69,999 (this category is very flexible)
* Picture Books - 500-S600 words (32 pages)
* Western - 50,000-80,000 (60,000 is a good target)
* Memoir 80,000-89,999 ( 70,000-79,000 is not a terrible thing)