What is the grind size for light roast coffee? And dark roast coffee? How to find the correct grinding size for coffee?

A dark roasted bean is more brittle and breaks easier into smaller parts, and the coffee will extract easier. The light-roasted bean is denser, less easy to break, and doesn´t extract as easy. For best taste, I suggest you grind the light roasted finer than the dark roasted. For professionals, I would recommend a Coffee Particle Analyzer that measures both grind size and surface area.