2002 Pontiac Grand Prix Catalytic converter

Hello, I had a direct fit catalytic converter installed last November after the after-market one the shop installed continued to have problems. Since then engine light goes on and off for periods of time and codes show cc issues, but shop says fine. Dealership told me recently that there are 2 sizes of converter and the shop had installed the smaller size which is what's causing the problem. Called the shop who were not all that helpful. Don't want to buy a new one (again) or spend more time & $ getting this diagnosed, so am wondering how much of a problem it is to run the car with the smaller unit (i.E, will more damage occur or am I fine to leave it). Thanks!