Assuming its not an S, you should be running a 225/40 and 285/30 setup.

 

ps2 265 225 was never a stock setup. not "n" tires anyway. (anyway best thing out there imho, what I run)

conti sc and pirelli were stock and both are the same exact size of the ps2. (diameter)

25.1 front
25.3 rear

on a 7.5 wheel front, and a 10 rear, which results in a bingo !! smaller !! diameter on the rear.
7.5 beeing relatively narrow pushing/squeezing the tire more vertically out. the 10 beeing on upper limit for a 265, flattens the rubber.

also take the rapid wear of the rear compared to the front into account wich will accentuate this minimal difference rapidly.

to the original poster.

justin is right, if your car is a 2004 c4 225/40 285/30 would be stock for summer. 225/40 265/35 is proposed for winter rubber

 

I hate to be argumentative, but I really don't think a narrow wheel can change the diameter of a tire, hard to strech those steel belts. I agree that as the rears wear they would become slightly smaller. However, we still see that on early C4's the rears started as a larger diameter, and on later years the rears started as a smaller diameter.