Exhaust piping size?
 

I bought a 07 FA5 and it came with an exhaust system already. I'm trying to replace catback but I don't know the size of the piping. So the question is with the current catalytic converter, will it fit let's say if I get the right piping size?

Also, If I can't tell the piping size from the converter, would it accurate if I get under to measure the pipe? D=C/Pi?

Or is there an easier way to do all this? Maybe there's a badge somewhere on the converter that tells the size? Any help would be appreciated.