This is what a 03 with 18" and a 2" drop looks like:

This is Team VOP's corolla who joined us on the last meet.
I say go with 17" since 18" are vey aggressive. Check out the TireRack.com or tires.com They have fitment guides that show you what you car looks like with different wheels.
Tire Rack recommends 215/40 18 for 18" tires and 205/50 17 for 17" tires. Let us know what you find!