17's are the biggest you can get for the ZR as a factory fit option. Imo, they are big enough.... Well for me anyway. 
I'm not the expert on this, but from what I recall, if you go bigger then 17's you'll have handling problems and your car will tramline. ???
I'm not the expert on this, but from what I recall, if you go bigger then 17's you'll have handling problems and your car will tramline. ???