From the Early to Rise newsletter
Flash Fiction (sometimes called Microfiction) - a work with all the elements of a story that takes place in 750 words or less (as defined by James Thomas in Flash Fiction: 72 Very Short Stories). Consider this example by Ernest Hemingway, just six words long: "For sale: baby shoes, never worn."
Nanofiction - a story with at least one character and a discernible plot that is exactly 55 words long
Short Short Story - a story that's under 2,500 words
Traditional Short Story - a story that's under 7,500 words
Novelette - a story that's between 7,501 and 17,500 words
Novella - a story that's between 17,501 and 40,000 words
Novel - a story that's at least 40,001 words long (Source: TheValve.org)