Wheel Question? I Need Help??

I was looking at the site Buywheelstoday.com and I clicked Chrysler 2005, Then i Clicked 300C, and on step 4 there asking me { Step 4 of 5 Select the original factory tire/wheel size that came with your vehicle.} Anybody know what the original Factory Tire/Wheel Sizes Are? I put in this one - 18"
225/60R18
Both
C (RWD/AWD)

I think thats the right one that comes standard on the 300c, After i clicked that 1 it brings me to step 5 and i put in what size i want to buy im thinking either 19 or 20, THEN after that it has a current front and rear for my car which already has 255/45R19 You Can Change it so is this right or do i change it??