Larger (coarse) grind sizes need more contact time with water to extract optimum flavor. Smaller (finer) grind sizes need less contact time to extract optimum flavor. In general, we use a coarser grind for longer brewing times (4-8 minutes) and a finer grind for shorter brewing times (1-4 minutes).
A coarse grind lets water pass through quickly. A fine grind slows the flow of water.
One effective way to communicate and evaluate grind size is how it feels to the touch. Take a sample of ground coffee and rub it between the thumb and fingers. Coffee is ground into particle sizes ranging from something resembling kosher sea salt for coarse, sand for a medium grind, and powdered sugar for a fine grind.
Ultimately, it all comes down to taste. Taste is very subjective. Assuming all the other essential brewing variables are in line simply ask: Does the brew taste bitter, astringent, or too strong? Adjust the grind coarser next time. Does the brew taste weak, watery, flat or sour? Adjust the grind finer next time. Small adjustments make big differences. Keep the adjustments small at first then continue adjusting to find the sweet spot.
To put all of this information together in a way that applies to different brewing methods refer to the grind recommendation chart for a good starting point on your preferred brewing method. Now you should be able 'dial' in the grind and enjoy some seriously tasty coffee. Enjoy!