In this sense, the sequel is not a follow-up novel or story. Rather, a sequel is a unit of reflection, introspection, emotional analysis, and planning that occurs in response to the action of the preceding scene.
In our preceding example, we left our protagonist stranded on a roadside, his car in a ditch. Now, consider how that would make you feel. What emotions come to mind? Frustration? Anger? Disappointment? Maybe even a bit of fear if you’re far from home and don’t quite know where you are?
Now, consider what you might do in this situation. Would you hitchhike back to the main road? Would you walk along the dirt road to find a house to ask if you can use their phone? Or maybe you’d go into the nearby woods to find thick branches to use as a fulcrum and lever to free your car? All of these thoughts could occur to you, and you’d need to weigh your options to decide which one to pursue.
As the protagonist weighs his options, he still needs to remember his date. He wants to get strawberries (the external superobjective) for his date’s dessert. So, whichever options leads him back to the supermarket fastest will doubtless be the option he chooses.
This is the sequel, then: Emotions lead to ideas lead to choices. The choice the protagonist makes leads to the next scene which, again, is a unit of action.
Sequels may get shorter as time goes on and the action builds. There’s no real hard-and-fast balance between scene and sequel, but generally sequels are shorter unless you really want to spend a lot of time in your protagonist’s head.
It should be noted that longer sequels don’t translate well onto screens. Scripts are all scenes and sequel content has to be expressed verbally or through some other fashion. Ever been excited to see your favorite novel turned into a film only to find that you hate the film? Look at the sequels in that novel and you’ll see what I mean.