One common question among writers (or would-be writers) in the initial phase of a project is: How do you know if an idea is a short story or a novel? I think the answer is that you should plan for every idea to be a short story. Try to make everything you write as short as it can possibly be without losing anything essential.
Naturally, some stories will necessitate longer word-counts, with more characters and more scenes to execute the idea, while other ideas can be fully explored in a short story—even a flash fiction story. In general, stories (of all mediums) tend to be too long and would benefit from trimming. Stories are best-received when consumed in one sitting, as Edgar Allan Poe said: “If any literary work is too long to be read at one sitting, we must be content to dispense with the immensely important effect derivable from unity of impression — for, if two sittings be required, the affairs of the world interfere, and everything like totality is at once destroyed.”
November is National Novel Writing Month (NaNoWriMo), something I have never participated in personally since I’m always writing every month anyway. It’s great that NaNoWriMo inspires more people to write fiction, but I think it sends the wrong message by valuing word-count over quality. National Fiction Writing Month (NaFiWriMo) or National Story Writing Month (NaStoWriMo) might be a better idea. Just write a fiction story, and if it’s only 35,000 words, that’s fine. You did not fail by coming short of 50,000 words. A concise novella is better than a verbose novel. The same problem arises in many literary markets today that pay by the word, incentivizing writers to be more long-winded than necessary in their prose.
The worst place you can be as a writer is planning to write a novel and discovering in the process that it’s too short, then forcing yourself to elongate it to meet the expected length (40,000+ words). This problem usually arises in the second act of a story, where you add scenes and try/fail cycles just to get to the third act. But it’s just filler. Sometimes an idea for a story is only two acts—a first and third—and there’s no need for that padding in the middle. The second act is where a lot of writers run into trouble, and their story tends to drag because they’re just feeling the need to make a story longer to hit the length of a novel.
If, on the other hand, you write with the intention of your idea being a short story from the start, you will avoid ever having to add filler. Your stories may be shorter than you would have anticipated, but that’s better than it being longer than necessary. Many of the ideas that you intend to be short stories will inevitably spiral into novellas or even novels, but they will become that length naturally. You cannot help but make them longer. That’s how you know if an idea is worthy of a novel: You start by assuming it’s a short story and the novel will force itself into existence. A novel is a short story that becomes too long.