I'm not an electrician, but I think it is the smallest wire in the circuit that determines what you can do at the panel. I mean, just because you might have a 12g wire in the panel, maybe it's only a few feet long and is then connected to a smaller 14g wire for the rest of the circuit. DIYers did silly stuff back in the day too, not just now. If you don't know what size to screw in, a licensed electrician should inspect the entire circuit to figure that out. Overloaded wires are one of the major causes of fires, so don't GUESS and don't ASSUME everything downstream of the panel meets code. Here's one time to pay the pro.