Pipe size for 1999 Passat

i'm planning on getting a magnaflow muffler for my 1999 passat GLs, what size do i need and is there any other info that would help in selecting the right muffler?

I think the passat actually has 2 mufflers in the system. There is one in the middle of the car and one in the back. If you are going for better sound and performance you may want to make sure you are changing the right one. I change the back one on my 97 jetta and it didn't sound much different or perfome any better.