What Is a Scene in a Novel
A scene is the unit where a value changes for someone. Robert McKee's definition in Story (1997) is the one worth keeping in mind: if nothing shifts between the start of a passage and its end, from positive to negative or the reverse, that passage is activity rather than action, however much happens in it on the surface. A scene can be quiet, short, and set in one room. What makes it a scene is that the person walking out of it wants something different, or knows something different, than the person who walked in.
Where the confusion comes from
It's tempting to define a scene by its container: a location, a stretch of time, a change of setting. That gets the packaging right and the substance wrong. Two characters can talk in the same kitchen for ten pages and pass through several real scenes, each one turning on its own value shift, or they can walk through an entire city and never leave a single scene, because nothing about their situation actually changes along the way.
How this shows up in a real novel
In Great Expectations (Charles Dickens, 1861), Pip spends much of the book believing his mysterious benefactor is the wealthy Miss Havisham, and that she means him for Estella. The scene where the convict Magwitch reveals that he, not Havisham, is the source of Pip's fortune turns that belief inside out in a few pages: Pip's value shifts from quiet, secure hope to a kind of horror at everything he's assumed and how he's behaved because of it. Nothing about the room changes. Everything about Pip's situation does.
While you're drafting
A scene that feels flat is worth checking against this one question: what does the person leave with that they didn't walk in holding? When the real answer is nothing, that's usually the actual problem.
Questions
Is a scene defined by its setting? No. A setting is where a scene happens, not what makes it one. The same room can hold several real scenes, or none, depending on whether anything actually shifts inside it.
Can a scene be very short? Yes. A scene's length has nothing to do with its definition. A single exchanged line can carry a full value shift, and several pages of talk can carry none at all.
What's the fastest way to tell if a scene is working? Name the value at stake, positive or negative, at the very start of the passage and again at the very end. If those two answers are the same, nothing in the scene has actually happened yet.