Aftermarket performance muffler on Impreza?

I'm new to the whole "tuner" scene, so bear with me: I'm looking into buying a flowmaster or Borla performance muffler for my 2008 Subaru Impreza 2.5i. One thing I'm concerned about is the inlet size of the aftermarket muffler I'm buying. Does anyone know what size what be compatible with my car? I'm thinking maybe 2.25 or 2.5 inch, but I'm not sure.