I don't know if Tesla can make a car much bigger than the X, though. The drag coefficient of the X is 0.24, and the smaller size drives a lot of that low number. The larger SUVs all have drag levels in the 0.35 or higher range. That would cut into range significantly on an EV.
No. The drag coefficient, Cd, is a way to compare the resistance of different shapes independent of size. The cross sectional area is explicitly separated from the Cd. The X's .24 is the result of the smooth curves, not the overall size.
Your basic point is correct, though, because the aerodynamic drag is a function of the cross sectional area as well as Cd.
Walter