Finding the Right Coffee Grind

In short, it's about extraction. Too fast, and you don't get the right flavours and too slow and you'll get off and bitter tastes.

With a setup like the one you describe you can't change the speed the water passes through coffee (unlike espresso, for example) so you control how long water spends with your coffee (and the surface area of contact) by the size of the grind.

The water will tend to stay and mud up super fine grinds. If you were to try to filter through turkish grind, you'd be waiting a long time and might taste something harsh.

Too coarse of a grind and the water will run through and not pick up enough (low extraction).

Aside from colour, taste and aroma, what you are looking for is how long it takes for water to pass through the coffee grind. I can't tell you how long for your specific setup, but if you're going slower than 20 seconds per ounce of water, it's likely too fine of a grind. Also note that the longer water spends with coffee, the more caffeine it'll pick up.

Finally, if you can, keep an eye on the drips as they leave the filter. When the drop or stream is no longer cloudy and starts looking clear, that's about when you've extracted what you can with that grind (both amount and coarseness).

Coarseness vs fineness will have a large amount of impact on strength and mouthfeel of the final product. The finer the grain, the greater the surface area that is exposed to the water you are brewing with. That means in a fine grind the water will absorb far more of the coffee's flavor. It also means more essential oils are being absorbed out of the coffee, so a more luscious mouthfeel and stronger aroma. It also means that longer exposure can quickly turn the coffee bitter and astringent, especially with lighter roasts (the grassy origin characteristics quickly become lemon tartness and bitterness).

For a pour over method, especially one without a paper filter (paper tends to absorb essential oils, hence the difference in mouthfeel between, say, a french press and a drip machine), a fine grind should be a great place to start. If it's too tart or bitter because the water is taking too long to drip through, gradually increase the coarseness until you find the grind you feel is most pleasant. HTH.

Also, I've never worried about silt in coffee. We can eat beans whole and the silt never causes a problem, and the silt never causes a problem with Turkish coffee, so I don't see it really being an issue.

I agree with Matthew's and MandoMando's answers. Here I just wanted to add some experiences that I made with chorreador coffee in Costa Rica, as your question mentions it.

The grind that people used in CR was very similar to the one I would use with a paper filter for drip coffee, maybe a tiny bit finer sometimes. Even though you are stirring while pouring the water, some "socks" will get easily clogged with too fine grind, and almost nothing will get through.

Sometimes I also saw people just putting the "sock" to sit in the mug for a while (like a tea bag) when they wanted a stronger coffee.

And here I just quickly took a photo of a grind purchased in CR, just as a reference point for where you could start your experiments:

for scale: I used 5mm graph paper and added a quarter

The smaller the grind the more surface area you expose. In theory this means that smaller grinds will produce stronger coffee. For some brewing methods you need to compromise a bit to keep the grit out. French press usually needs as coarse of a grind as possible so your coffee isn't dominated by grinds. Gritty coffee isn't an issue for paper filters so it can afford a finer grind. However if you grind it too finely, then the coffee won't drain properly and it will spend too much time extracting the coffee, possible leading to bitter flavors getting extracted. Espresso machines can use a fine powder because the water is forced through under high pressure, sometimes as high as 20 bars.

So to sum up, the grind size is a balance between how much coffee flavor you want to extract and how much time the water spends trickling through the coffee grinds.