what size muffler and catalyic converter for my car

Go to a professional muffler shop, they know..

You could I suppose replace this yourself, BUT, I would rather put the responsibility on the professional in this case, they are catalytic converters, you want this to be PERFECT.

Besides, do you really have welding equipment and skills do do this? It's mostly a parts cost and some labor, trust me, hire a professional for this...