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The SFWA (science fiction writers of America) established the following guides for the judging of awards such as the Hugo and Nebula awards, and is generally considered a reasonable standard:
Flash: up to 1k words
Short: up to 7.5k words
Noveletta: up to 17.5k words
Novella: up to 40k words
Novel: anything beyond 40k.
Note that any publisher has the right to set their own standards, so they might consider a 40k word story a 'short' and nobody can really argue with them. The magazine, Asimov's Science Fiction, accepts shorts up to 8k words.The same goes for contest rules - each contest can set its own rules. Note that NaNoWriMo sets the aim to write a 50k word novel in a month - clearly suggesting they consider 50k words equate to a novel.
This also means that you can define your own guidelines. If you write an 8k word story and declare it a novella, nobody can argue - unless you send it to a publisher or enter it into a competition where they think different.