How long is a story? The same length as a piece of string, right? Twice as long as half its length.
I know the generally agreed upon word counts for what constitutes flash fiction, short story, novellas and novels, but when get an idea, how do we know whether it will be a short story or an epic tale told over three books?
Obviously certain ideas lend themselves better to shorter tales than longer ones and vice versa. Sometimes it's quite intuitive. You just know that an idea, even if it's only the bare bones of one, requires a novel to be told or only a few sentences.
And I think you need to stick to your gut feeling initially, at least when planning it. Trying to compress a novel into a short story can result in a piece that reads like it's screaming out for more words. And not enough words won't do it justice. But trying to turn a short story into a novel can stretch a great idea to breaking point. Even by beefing it up with subplots you risk diluting the narrative. Too many words can tear apart a piece.
But then as you write, I think you need to write the length that's right. You might start off with the plan for an 80,000 novel but you soon reach 20,000 words and feel you're already over halfway through. Conversely you might find yourself needing 15,000 words extra for a short story. Don't necessarily constrain yourself to what you originally set out to do. It can be heartbreaking finding your piece won't be as long as you intended, or is far longer, but a story needs be the correct length - it shouldn't be the length you want it to be. It should take as many words to tell as it needs, and not one more