Medium-fine grind size is the best for Moka pot coffee. A finer grind size will result in an over-extracted cup of coffee, unpleasantly bitter. If the grind size is too coarse, you will notice because it will taste sour and, in some cases, bland. Coffee brands tend to use a medium-fine grind size because it's the best for home coffeemakers and Moka pots. Still, if you aren't sure, check if your ground coffee feels like table salt to touch. Espresso and Turkish-style blends are ground extremely fine. For this reason, they aren't appropriate for Moka pot if they come ready to brew.