Size of pipe from engine??

[Frey7190]

I am looking at replacing my A4's CAT... I have this really bad rattle from underneath my car and I think its A.) CAT of B.) Heat shield so im going to hope its the CAT. I want to replace it with a magnaflow, but they come in three different sizes from 2.25" - 3". What is my stock pipe size so I can buy the right one.. Also if I did switch it out.. what would I notice going to a magnaflow cat from stock... Would I get the CEL??

EDIT: Sorry I forgot to mention mine is a 3.0L V6 -- do the 3.0's have 2 cats??

[Frey7190]

Bump