If I put 18"'s how much can I drop the car? What size is good for racing?
you need to get your mind right. If you want to race
you want small wide tires, as in 15-16s not large ones. When you throw 18s on you're slowing yourself down.
As far as the drop: With 18s the drop depends on what kit you have. For example 18's with an R2 or Blitz kit doesn't have a lot of space to drop. But a safe drop would be 1.5" or less. Anymore you run the risk of bottoming out.