6/3 romex can handle up to 55A or a 44A EVSE, but I don't know that you can find a 55A breaker or an EVSE that can be set to 44A. Practically speaking, you will have a 50A circuit that will support a 40A EVSE.What kind of wiring would you suggest? I just bought 75 feet of this 6/3 romex stuff, ran it and installed charger for my RAV4 EV - which will be fine. But I didn't run the other line for the Rivian yet. My line is about 30 feet (hince why I bought 75 feet for 2 lines). I've got a big enough box for both - brand new build and made sure of that.
If you want to run the Rivian EVSE at 48A it will require a 60A circuit.