Okay. What you want for the rear then is a 10 or 12mm then. There are those that might tell you to go bigger but this would a) require fender rolling and b) ultimately compromise the utility of the wagon. what good is the wagon if you can't haul things in it because your car will rub?
If I were you I would go with 5mm up front and 10mm rear and be done with it. This will give a very full look to the wheel wells without compromising the utility of the car. Good luck!