stupid question about exhaust

[Black Box]

Im going to getting my exhaust done. I already bought the super turbo dynomax muffler w/ 2.25 in/out. Now I need a universal cat since mine is rusted through. Im looking at the magnaflow universals and they are weld on. So, my dumb question is do I need the same size in/out as my muffler if I want to run 2.25 piping or do I need to get a bigger or smaller in/out size?

[dino245]

Run the 2.25 through the cat.

[dhirsch]

I've got a magnaflow universal on my 318.  before i had it I couldn't pass emissions and once I put it on i had no trouble!  Very good product