Researching this myself: 19" vs 20" aftermarket. My OEM 18" are 26" diameter and the 20" options were 26.3" front and rear 26.2". Not sure how much room there is to play, but I was using those numbers as the max height in my research.
First and foremost I feel the car needs WIDER treads all around. So far the issue I'm seeing with the 20" is if you go wider than 255's in the rear then your looking at a 25 series tire it seems. I had the 295/25-20 Nitto Invo recommended which is 25.79" diameter.
Whereas with 19" there are plenty of 26" diameter tires wider than 255's. There's even 295/30-19 options that are 26" (Michelin & Bridgestone). Keep in mind I'm looking at mainly 'Max Performance' summer treads.