How do you know when it’s a novel not a short story? I’ve turned this idea every which way i can think of. Come at it from different directions. It won’t coalesce the way my stories usually do. I’m beginning to think i’m trying to fit an elephant into a shoebox.
This can often be a difficult call, so don't be annoyed to have come up against a rough side of it. Most of us do eventually. (And for the moment I'm going to avoid the "Define a short story" end of things.)
First of all, if I was in a difficult mood, I'd just say, "Why are you sitting here wasting time talking to me? Start writing. If you cross over 50K words, it's a novel." (Though in this neck of the woods, both @petermorwood and I would have been embarrassed to call anything shorter than 70-80K a novel. Granted, we both would have come of the time when dinosaurs walked the earth. [And possibly wrote novels.]) 😏
...But I'm not in a difficult mood, and anyway, that wouldn't be helpful.
Let me offer you a small spread of (theoretically) more useful possible diagnostic strategies... because the one I routinely use, the "weighing the story in the hand of the mind" technique, relies on solving for the interactions of a large sheaf of possible qualities against one another—not to mention needing many years of experience—and there's no way it can be relied on to be of any use to you.
(a) One of my editors said to me once that a novel, even if it's one in a series, should routinely describe "the most important thing yet to happen in [the main character’s] life (or should at least be a good candidate-event to be described that way)."
That's... not a terrible place to start constructing an answer backwards from. Does what happens to your main character(s) in the work you're considering fit that description? Then you may indeed have a novel there. Look into that.
(b) Another set of criteria to consider: do you have a lot of characters who're all interacting to drive your plot, but in ways that can't be boiled down to (and explained to one another) in just a paragraph or so?
And, sort of (b1), do your characters have individual motivational or "crisis" arcs running that are going to inevitably pit them against one another? Maybe even repeatedly? (Not that inter-character drama isn't vital even at what might seem quite low levels. But here I mean big personal drama, or crisis-dependent payoffs that would routinely bid fair to derail the forward thrust of the plot until they're successfully handled.) ...A situation like that argues that you're going to need the room a novel allows you to get everybody adequately sorted out.
(c) Anybody('s character) got an archenemy? Like, an actual one? Those routinely need longish arcs to be properly perceived, engaged with, and defeated. (Especially if they're actually worth anything.) Heinlein uses what may well be someone else's line on this subject, and says "To every cat, a fine rat." Your archenemy must realistically and engagingly challenge your main character: otherwise their defeat will be hollow. For a Holmes, you need a genuinely threatening Moriarty. For a Superman, you need (as I see it) a properly dark Darkseid. (Whom I've always been oddly fond of. When correctly written, Darkseid has dignity to him, and a sense of nasty evil gravity.) But to build that relationship, and its destruction? You want a novel's length, and time to explain what happens and exploit the results.
(d) The opposite pole of that diagnostic: anybody got a Mission? I mean, a BIG one? To my mind, that's one of the easiest novel "tells" there is. The kind of Mission that makes you suck your breath in à la the Impressed Dishwasher Repairman and go "Yeah, wow, this is going to cost you..."? That's a novel. Or so it seems to me. Problem with no quick and simple solution, and one that'll make your reader sit still and keep turning page after page (for hours or days) as they wonder how you're going to pull this off? Novel.
And one more to be going on with. (e) Got a love story? ...And I don't necessarily mean a romance. ...Might be a novel there, because it's hard to do that kind of thing well in a short story. (Unless you're Ted Sturgeon, who [alas] seems not to be writing at the moment.)
...Seriously, and with deep respect for a genre that dominates publishing at the moment: the gravity associated with the modern Romance genre is so powerful that it's too easy, when faced with characters in love, to be tempted to sink into the genre's depths and its tropes when there are so many other roads to walk.
You can have a strong work about all kinds of other subjects that has a love story at its heart, and therefore nonetheless needs to be a novel because that's the only way the love story at its core can be given the attention it's due. Forgive me if I have a well-known soft spot for this kind of thing. But have a look at your story and see if inter-character business of this kind might be lurking at its heart. If so: novel.
Anyway: try examining your prospective project in these lights, and see if you can find an answer that makes sense.
