What size of wire do I need to run from 200 amp meter panel to 100 amp sub panel in detached garage 215' away?
1 AWG uncoated copper, or 2/0 aluminum. Unless you're not concerned with power quality, then you could do 3 AWG copper or 1 AWG aluminum.
Using copper on such a long, heavy run is throwing money away. AA-8000 aluminum is the way to go.
When you punch into the voltage drop calculator, and you say 3%, don't stop there. Go back and try some other numbers. For instance I put in 20% and the answer changed. It said 1/0 aluminum and tells me I'll have 3.8% voltage drop if I do that. And 3.8% is perfectly acceptable. Sure glad I asked.
Why did it say that? Because 1/0 Al is the minimum legal size.