The short story has been referred to as an” apocalypse in a teacup.” It differs from a novel in that it focuses on style more than action and plot. There is dramatic and thematic unity.
It can be as profound as a novel with fewer characters. The concentration is on voice, characterization, structure, and dialogue. Changes of setting are fairly unusual and, in general, short periods of time are covered.
Only the essentials exist in a short story. It is urgent and acute. There are no repetitions, lengthy descriptions, extended passages or long dialogues, wasted characters or extraneous scenes. The author writes as a poet, using implication, careful selection and compression to achieve the required effect.
Conventionally, something with 40,000 words + is considered a novel. A novella would have approximately 17,501 to 40,000 words. A short story can be from approximately 300 words (sometimes called short-short, sudden or flash fiction) to 5,000 words, even 7,500. These numbers are extremely fluid. It’s largely a subjective matter for which editors and publishers assign arbitrary numbers based on their needs and available space.
A classic short-story shape has a definite arc--gradual build-up to a crisis point, followed by a falling-off. Main stages can be the initiating incident, complication and conflict, rising action, crisis and falling action.
Ultimately, you must let the story dictate its length.