the biggest tire size?

what is the biggest tire size to be put on a 18x9.5 wheels they are foe the back of a drop top 02 z28 thanks

If you go strictly by the tire manufacturer's recommendations, most manufacturers do not recommend anything larger than a 285/35-18 on an 18x9.5" wheel. They would typically recommend a 10" wide wheel for a 295/35-18, but you might be able to get away with that.

Whether or not the tires rub on anything will be determined by the offset of the wheel.