The key is the overall diameter of the tire not necessarily the wheel diameter. For example, 195/60-15, 205/50-16, and 205/45-17 all the same overall diameter and require no need to mess with the ECU when changing between these wheels and tires. Your goal should be to use an 18" tire with the same overall diameter as the 17" tire that came on the car.
If your car came with 215/45-17, 215/40-18 or even 225/40-18 are very close to the overall diameter of the 17's. You just need to take into consideration wheel width, tire width, and wheel offset to avoid potential rubbing and clearance issues.