People sometimes tell me that a longer sentence sometimes improves the flow of the prose. So I was wondering what they meant by that. How do you determine the flow of a prose, and how do you measure it? Is there a way to measure it, or is it something that's subjective like modern art? I am wondering, because there doesn't seem to be a precise definition for that term.
I haven't heard of the term "flow of the prose" before, but I assume they're talking about the pacing of your writing at the smallest level. You see, your story has different pacing throughout, and if you look at the major strides, you'd see a pretty smooth graph. Zoom in, and you see a few bumps here and there. Zoom in far enough, and you're now looking at the pacing of paragraphs.
At this level, pacing is set by the length of your sentences, and you determine the appropriate pace from the immediate circumstances (in omniscient narration), or from the POV character's experience of the immediate circumstances (in limited narration).
So, short sentences are good for action, of course. Long sentences are more for circumstances involving thoughtfulness and calmness. But for the most part, your sentences ought to medium-sized (and what medium-sized is depends on your target audience). If you have a lot of choppy sentences in succession, you can create a feeling of efficiency. Having a sequence of sentences getting increasingly shorter however? That creates a feeling of panic. Passing by long sentences and entering the realm of ultra-long, messy sentences that are composed of adjuncts; disjunctions and conjunctions, tangents, lists mid-sentence, way too many superfluous and unnecessary adjectives, and crazily qualifying them with adverbs just for good measure? Yeah, that can also create a feeling of panic, but a different kind; one happening in the mind, concerned with matters of the mind, and not immediate physical danger.
As for your question about subjectivity: yes. This is art. Anyone who thinks they've got it figured down to absolutes are wrong.