I think battery pack size is irrelevant since I would hope that various efficiency improvements will produce ever greater range with a constant battery pack size.
But if the intent of the OP is to ask how much range do I need, then the answer is the greater of 400RM or the maximum distance between SC's AND charging at about 250kW. 400RM would deliver 300-350mi of real world driving in consideration of wind, weather, etc. and at 70-75mph would mean 3-4hrs of driving time which is the point at which it's a good idea to get out and walk around, take a bio-break, etc. 250kW charger would get me back on the road in a reasonable period of time.
For reference, my 2011 Mercedes E350 Bluetec went 650mi on a tank of diesel and took 10min to "recharge".
There is little room you can improve on efficiency nor is there much room for lowering the aero. A bigger pack seems to be the only way to get more range from my point of view. I don't think a 250 mile is a sweet point for such an expensive car. It might be the sweet point of triggering sales of a certain EV. Model S is far from a perfect car, lacking some range being the most important one as I can see. I'd agree 650 miles is too long for most people, however due to the slow "refill speed" of EV, longer range is still desirable.