You know how many times I get asked, "What's the best length for my book?! I want a book publisher to think I'm the new Hemingway! The new Devil Wears Prada chick lit great! The–"
Calm down, silly Sally.
For a traditional old fruit publishing house collecting dust on their books, you might need rules. The rules now amongst publishers at all levels like myself and DIY self publishers are gone.
You can have a hit novella, short story, collection of short stories, or a novelette which is a short novella. Set your sale price accordingly.
"How many words is that?"
From the Science Fiction Writers of America:
Short fiction: under 7,500 words
Novelette: 7,500-17,500 words
Novella: 17,500-40,000 words
Novel: 40,000+ words