For espresso; you'll want a very fine grind, almost like powder. This is because espresso is brewed under high pressure, and a finer grind will allow the water to extract more flavour and caffeine from the beans. If the grind is too coarse, the water will pass through the grounds too quickly, resulting in a weak and under-extracted shot.
For French press or cafetière; you'll want a coarser grind, as the longer steeping time allows the water to extract flavour from the beans even with larger particles. If the grind is too fine, it can seep through the press filter and create a muddy, over-extracted cup of coffee.
For drip coffee makers; you'll want a medium grind size. This allows the water to pass through the grounds at a moderate speed, extracting the right amount of flavour and caffeine without over-extracting. A finer grind can result in a bitter, over-extracted brew, while a coarser grind will produce a weak and under-extracted cup.
In summary, the grind size has a major impact on the strength and flavour of your coffee. To get the best results, be sure to match your grind size to your brewing method. Whether you prefer a strong and intense espresso or a mellow and smooth French press, the right grind size can make all the difference.