How short can a story be in order to be sold as intellectual property? I was wondering about this, because Starship Trooper was adapted into a film, and the film adapts a short mention of bug-like creatures that had nothing to do with the story. Knowing that, I was wondering how short a story (novel, comics, short story) could be in order to be sold as intellectual property. Since short stories were also adapted, like Minority Report, I would say 3,000–5,000 words is enough, and 3,000 words is not even equivalent to an arc in comics books since so many things can happen in a single panel.
Anything you write that is unique, even a 20 word poem, is copyrighted once you write it, and your intellectual property (unless it is a work product for an employer or you have an agreement to surrender your copy rights.)
Thus there is no technical limit. A single made up word can be your intellectual property. Ultimately, any non-obvious decisions on "unique enough" must be decided by human judgment, specifically a judge or jury.
That is who will decide if your word, poem, or short story is creatively unique enough that somebody else almost certainly used your creation without your permission, rather than having invented it themselves.
This is also why you cannot just take a Stephen King story, go through and change all the names and use synonyms and such, and sell it as your own. If a human jury can recognize that your story is a just a rewrite of Stephen King's story, you lose.
The decision is up to human judgment, not technicalities of how many words and whether it is an exact copy or not.
And that is why Movie studios, typically with both budgets and expected revenue in the tens of millions (and up), will be quite careful in formally paying for rights and giving fair attribution: Like "Based on a Stephen King novel."
That's not just to sell tickets to Stephen King fans (of course it might), it means they paid him for the rights to his original creative ideas. They listen to their full time legal teams and license the idea to eliminate any chance of copyright violation, which could cost them many $millions$, even if their film doesn't earn enough to pay their expenses in making it.