A medium-coarse grind (similar to rava/sooji texture) works best for filter coffee brewing.
When the coffee powder is too fine, it can pack tightly and clog the filter, slowing the flow of water. This makes the brew take longer and allows the water to lose heat, which reduces extraction efficiency.
Very fine grinds can also cause tunneling, where water finds a narrow path through the coffee bed instead of flowing evenly through it. This means some coffee is over-extracted while the rest is barely extracted.
A medium-coarse grind allows water to flow evenly through the coffee bed while maintaining the right brewing temperature. The result is a strong, balanced decoction with better flavor and consistency.