I'm looking to get a set of rs4 wheels, i beleive they are 18x8.5 what is hte biggest set of tires that i can put on there??? Without causing other issues rubbing and whatever else...

My car has not been lowered, but it will be...

 

225/40/18 is best... you can try a 235/40/18, but it's risking it -- you might end up having to roll your fenders/cut out the wheel well liner.