I've been looking into getting a Magnaflow muffler but I don't know the exhaust pipe diameter so I'm not sure which one to buy. Also, what is the size of the muffler? Is it 4x9 or 5x8 inch? Anyone know?

https://www.magnaflow.com/collection...rmance-muffler

Plus, which muffler should I get?

 

So you want to install an aftermarket muffler in place of the stock one, without replacing any piping? Are you sure the existing piping is in good enough shape (not rusted out) to be able to weld to? Be mindful of the in/out locations for the muffler you're looking at the try to mimic the stock in/out locations (side or center). As for measurements just go outside and measure it yourself. IIRC stock piping is either 2" or 2-1/4", but again, verify it yourself with calipers.