FYI a catalytic converter heats up to do its job, and depends on a certain amount of exhaust flow to get hot enough to react.
If you use an oversize catalytic converter, it may never get hot enough and won't control emissions and could eventually clog because it's not "burning" off the residual HC's. A converter that's too small could also get too hot, but I don't know anyone who is going to try to put an undersized converter.
Be prudent in selection of any non-OEM catalytic converter if you have to pass emissions (and can't just disable the rear O2).
