Adult trade fiction should be between 80,000–120,000 words. Romance can be shorter and science fiction longer, as others have said. Children’s books usually range from 28–32 pages with few words. When an editor sees that an adult manuscript is more than 120,000 words it is an immediate red flag that he works probably needs some major editing. There are always exceptions to the rule. These numbers serve as broad guidelines, but children’s books get more specific, as seen below.
I have heard it said that the book should be long enough to tell the story. Most commercial books have about 12–15 chapters of 20–25 pages per chapter.
Literary Rambles has a good breakdown for children’s books:
  • Board Books: 0 - 100 words.
  • Early Picture Books: 0 - 500 words.
  • Picture Books: 50 - 1,000 words. 1k is pushing it.
  • Nonfiction Picture Books: 500 - 2,000 words.
  • Early Readers: 200 - 3,500 words, depending on age level.
  • Chapter Books: 4,000 - 10,000 words.

Hi-Lo Books: 500 - 50,000 words, varies greatly depending on age level. A large number fall between 500 - 20k words. Some 60-90k YA books get classified as Hi-Lo, but I don't think they were specifically written for the category. Middle Grade: 25,000 - 45,000 words, usually around 35-40k. Longer word counts allowed for fantasy, sci-fi, historical. Up to 60-70k is probably safe (though there are even longer exceptions). Young Adult: 45,000 - 70,000 words. Longer word counts allowed for fantasy, sci-fi, paranormal, his today on fi and age level (I recommend researching seem to fall within the same range as novels for their age group).