The amount of resolution you need depends on what you’re trying to do. Let's look at a very simple use-case: counting.
So what resolution do we need to count cherry trees? In this case, we could probably use a 40 cm pixel, but several things affect this: tree spacing, cover crop growth, tree size, etc. A different orchard, or a different crop might require a different resolution. We might ask the question a different way: “How many pixels do we need to identify an object, and differentiate it from its surroundings?”
It’s hard to give an answer that applies to every case, but in both our examples 8 pixels were good enough to give a count.
How much resolution do you need for using radiometric data–like plant vigor? Once again, it depends on what you’re trying to do.
So if you want to see basic patterns in vigor over this vineyard, a 1-meter pixel might be adequate. If you want to determine vigor along each trellis, a 40 cm pixel would work. If you want to see variation at the vine-level, you’ll want a 20 cm pixel.