The grind of the coffee controls the ratio of surface area to volume, which is one of the two factors controlling how quickly the coffee is extracted into the water (the other is temperature). If you are using an electric drip machine, you will not be able to control the temperature or the time, leaving the grind as the one factor you can control to vary the coffee extraction. This is not necessarily a bad thing — it allows you to vary one parameter while ensuring that the others are repeatable while you do so.
It is not practical for most people to directly measure the size of their ground coffee particles, nor are there convenient tables to look up the desired size as a function of time and temperature even if it were. Thus, coffee grind is set by a process of trial and error, knowing only that one setting is finer than another, but not by how much. Select a desired coffee concentration, use this to calculate the amount of coffee to use, and try brewing it. If the actual concentration is lower than desired, the coffee must be ground finer; if it is higher than desired, the coffee must be ground coarser.
Of course this requires both a repeatable grind setting and the ability to measure the concentration of the final coffee. For the former, a burr grinder is desirable; any number of them exist, with the Capresso Infinity being one of the least expensive that is still of acceptable quality. For the latter, you can either evaporate all the water out of your coffee and weigh the remaing solids, or buy a refractometer. If the dedicated coffee refractometer is too pricy, you can also buy a cheap 0-10 Brix analog refractometer, and convert Brix to approximate dissolved coffee solids by multiplying by 0.85. You can also just drink the coffee and decide which version tastes best!