The BMW i3 has different wheel sizes front (5.0 wide) and rear (5.5 wide). The 2018 also comes in an i3s model with 5.5x20 front and 6x20 rear (although I don't know any other details - which is why I am here). I also see nothing here about the offset(s).
5.5x20 ET336x20 ET39
Wheel for Winter tire 19 inch for BMW i3 2018
...not sure on what’s the best / biggest / widest / maximum tires or rims will fit your car?
Write about the tires you are currently driving on.