One way to define prose genres is by length.
Short short story = less than 2000 words
Short story= less than 7,500 to 20,000 words (depends on whose definition)
Novella= more than 7,500 words less than 40,000 words
Novel = 40,000 words plus
Please note that the short short story is most often the students’ favorite. Think of it as the commercial break and you won’t go far wrong.