I'm probably not the one to say this, having no official position here, but my understanding was that the limit was ten thousand characters (about two thousand words). I know that stories under 10k are sometimes accepted for posting to the Library, although not always, and I don't know just how the exceptions are determined.

Jinn does know, however, and he is very approachable and amazingly helpful.

Also, there are places in the Community section of the Forum where people (Mobius, Lady Amanda and others) have posted very short stories as posts to a thread. There is no formal review process for thread posts, but you'll probably get some kind of feedback (probably, "More! We want more!").