Twitterature: Up to 140 characters^
Flash fiction: Up to 1,000 words
Short story: Up to 7,500 (5,000) words
Long short story: 5,000-7,500 words*
Novelette: 7,500- 17,500 words
Novella: 17,500 - 40,000 words
Novel: Over 40,000 words
according to the Science Fiction and Fantasy Writers of America. But, as this Wikipedia article points out, varies according to genre. Jane Smiley says that novels are typically 100,000 - 150,000 in length.
What am I writing now, I wonder. It is composed of short stories and novelettes...with a novella infill. Maybe I shall call it a novelomerate.
*Categories in red are a refinement added later in response to Jim's comment.
^Also added later in response to Paul's comment.