Never design a water system without understanding what it is you wish to correct. Figure out what is in your water that you want removed by testing and then it is easier to determine what solutions will result in your goals.
Whole house filters are often 20-25 microns, but can be any of the choice you made....depending on.....what you are trying to treat.
Sorry, I deleted your original post. There was nothing wrong with it, I just hit the wrong button. You were asking what size micron filter you should use.