Sometimes I joke that I’ve written something as a story because it didn’t have enough content to be a novel or a poem. Novels and poems represent extreme ends of length for narrative forms (not all poems are narrative, but some are), and as a result I build them differently from each other.
Most of my narrative writing (including creative non-fiction) begins with an idea about a scene or a character. And if it’s about a character, it’s typically about how that character would act (i.e., what they’d do in a scene). If I only have a little bit more to say about that scene (or character), then I develop a short arc and put it into a story or write that scene as a vignette (or find a mini-arc and write it as flash fiction). If what I primarily want to convey is the feeling that the scene gives me, I’ll try to whittle/hack/burn the scene down to its essence and shape it into a poem. If I can imagine a lot more story, I may develop it into a rough outline for a novel.
Every once in a while, a poem comes up from a single line (e.g., in “Use”, the line “I know the use of the user” came from a bad Google translate result and stuck with me enough that I knew it was going somewhere). And every once in a while, a large plot comes to me at once and gets developed into a novel, but most of what I write starts out as a kernel of about 1,000 words.
How about you? How do you choose what form something should take?