Required generator power depends on what you want from it.
If you want it to provide all the power you could ever need, than it has to be pretty powerful - over 150kW or very close to motor max power or even over it, because of losses in wires and PEM. This makes little sense though. Get an ICE car and avoid the middleman.
If you want to have practicaly unlimited range (with gas refuels ofcourse) than it has to provide for average highway requirements or around 30kW. This sounds reasonable but it really isn't. We humans have our own constraints and have to sleep amongst other things ie. we have to take brakes. Generator doesn't and can keep running overnight.
So, it could be of lower power.
If it is only 10kW then your max range would double from 250 miles to somewhere around 500 miles when the car would switch into "very limited power mode" - you would only have those 10kW of power left. Enough for slow acceleration and about 30mph max speed. But after those 10 hours of driving your would be well advised to take a brake and maybe get some sleep. You could leave the generator running to charge up the batteries though. It would do that in some 6 hours. 10kW generator looks usable.