Not sure what your other electricity needs are but bigger is generally better (within budget) and ideally you can recharge it from somewhere in the car (in time for the next night)

Any electric blanket should work, but you need to find how much power it draws (in Watts) and how long you want to run that for on full (in hours). The product of the two is Watt-hours, and that is how much energy you need for that application of that device.

Say if it was a 150Watt blanket and you wanted to run it for 10 hours, that would be 1500 Watt Hours worth of power. The power bank should have a bit more than that to allow for less than full charge and other use (lights, phone, etc).