The question is somewhat open ended.... do you want to know the inner diameter of the pipes in question, or the outer diameter?
For the end pipes from the muffler body, you should be able to remove the shroud and make the measurements directly on the pipes. Both inner, and outer.
For the one going into the muffler, I'd be very surprised if the cross section of that pipe were any different from what is happening at the catalytic converter section of the pipe..... you can remove that part, and make the measurements you want, and apply them to the input to the muffler.
If someone has taken the OEM muffler off, and still has it laying around, it's that much simpler..... so far, I have not done so.