Flash fiction is a style of fictional literature or fiction of extreme brevity. There is no widely accepted definition of the length of the category. Some self-described markets for flash fiction impose caps as low as three hundred words, while others consider stories as long as a thousand words to be flash fiction.
You don't have time in this very short form to set scenes and build character.
2. Don't use too many characters.
You won't have time to describe your characters when you're writing ultra-short. Even a name may not be useful in a micro-story unless it conveys a lot of additional story information or saves you words elsewhere.