We don't have a good unit of measurement for fiction. Linguistics can easily point to words like 'clause' or 'sentence,' but those are frameworks. To put it another way, a sentence is the medium through which the 'stuff' of fiction moves. We can dance a little closer to what a fictional chunk is by looking at Stanislavsky's Objectives and Actions. Stanislavsky was concerned with actors internalizing the emotional states of their characters. So an actor develops objectives(desires their character has related to other characters in the story) and uses those to motivate actions(things they do on stage, either scripted or otherwise.) Both an objective and an action could be considered units of fiction. The novel as a medium is keyed in to this. A reader can expect to know the thoughts or internal struggles of a character, and then read the actions they take informed by those struggles.
Environmental detail and setting must also be considered in units of fiction. These can be very lengthy or very tiny. For instance, you can imagine opening up a short story: The first sentence is a single word.
"Darkness."
And your mind is off to the races. So, all this works out fine. A unit of fiction is a clause or sentence that elaborates on the fictional world the reader is building. You might say, 'Well, how is that any diferent from a clause or sentence?' Fair. We can hop over to Moby Dick for the distinction. Ishmael's exploits are all composed of units of fiction(whatever we call them.) But there are also sections of the book that aren't diagetic! For example, Melville's folios of the different types of whales, or the catalog of sacred albino animals across history. These sections are evocative, and might contribute to the mood, but they aren't "in fiction" in the same way as Ahab's monologues. Another parallel comes from Viola Spolin's "Improvisation for the Theater." Spolin lays out the idea of 'Space Substance,' the invisible medium through which the world is created for the actor and audience. Space Substance can be plucked out of the air, mashed up and rearranged into any shape. When you see a mime carrying an invisible bowling ball, the ball is made of space substance. The feast and food fight in "Hook" illustrates space substance as well. The stuff we create is to us what space substance is to the stage actor. To wrap up the analogy, you might say that sentences and clauses generally are the boards of the stage the actor walk on.
Part of the problem with coining a new term these days is that every dumb misspelling you could use for something is already a digital management solutions app. So for the time being we will call this stuff Diagetic Stuff, or Fictioning Stuff, and it's the stuff that causes you to hallucinate narratively. Now that we have this stuff, how do we pick it up and mash it in our hands to make a reality appear?
Let's hop over to music for a moment. When you hear classical music, it can seem overwhelmingly complicated. In fact, these pieces are much more structured than you might expect. For instance, let's take the Sonata form. A Sonata comes in three parts. Exposition, Development, and Recapitulation. In the exposition section, ti he composer lays out two contrasting groups of musical phrases. A Phrase in this case means one or two little musical riffs that reach a climactic point of pitch or volume. So we listen to all the phrases, one after the other in the Exposition. The next section, the composer takes all the phrases from the Exposition and starts messing with them. A phrase can be chopped up and rearranged, mixed with another phrase, reversed, stretched, squashed, inflected, recontextualized, repeated, and some other stuff too. This Development phase rises to a point of tension, which is resolved by the Recapitulation section. In this part, the two groups of ideas are recontextualized so that they work together in the same key. Instead of focusing on the differences between the phrases, we are focusing on the similarities. This is, to really simplify things, a three act structure.
That but this! This but that! The same way Space Substance can be manipulated, Diagetic Stuff can me developed. To follow along this parallel along, you can use this rough format to introduce and develop ideas you want to write about. The first section of a story is the place setting, character introductions, conflict introductions, etc. I think of this(to spin up yet another analogy) putting paint on the pallette. It's the collected material you're going to work with through the rest of the section of text. Then, development happens. In a lot of guides for writing, this is the mushiest and least helpful section. Here's how you do it. When you don't know what to write next, take two of those elements you picked for the Exposition section. So let's say your story is about someone who has fallen in love with their standoffish successful peer. You decided the setting for the scene was a park, maybe mentioned bees cruising around flowers earlier when you were setting the stage. Your thought process here is 'What elements can I mash together to create conflict?' You have local wildlife, as well as trying to impress someone. These two ideas are made of infinitely shapeable Diagetic Stuff. We can really blow up the significance of one bug into a threat, maybe one that puts the protagonist's desire to impress this person at risk. A bee crawls down your protagonist's shirt in the midst of the conversation. Conflict has been achieved! Now you have some material to work with. Trying to ignore a bee getting stuck in your bra could resolve in all kinds of interesting ways, from romantic to shame-inducing, maybe even a hospitalization. At this point you will probably find some paint you previously put on your palette that nicely compliments one of those options, and you do it again and again until you want to move on to Recapiltulation. For our purposes, Recapitulation is the resolution of tension. Put simply, you write out the consequences of the actions that characters have been taking. There is much much more to be said about resolving in this way, I'll write up something more about it. If you ask pulp writer Lester Dent, he would tell you that your situation should escalate toward physical violence, and then resolve with your protagonist barely surviving said violence. If the Exposition is your Causes, Recapitulation is your Effects.