For nearly all markets I deal with, flash fiction is from one hundred and one words to one thousand words. One hundred words and down is called micro fiction. A short short is from one thousand to two thousand words, and a short story is from two thousand to seven thousand, five hundred words.
Every length already has a term that applies.
I once sold a one hundred word story for an even one hundred dollars. Not a lot of money, but a very good word rate, and an excellent hourly rate, since I wrote the story in under twenty minutes. This is one huge advantage of very short fiction.