Usefull tool in link below.
http://scriptasylum.com/rc_speed/_top_speed.html
As a rule its better to use a higher voltage battery with low capacity, than low voltage battery with high capacity. Higher voltage is cheaper, lighter & puts less strain on the electronics. Doing some simple maths you can work out what battery size you will need for the race duration you are running the car.
I use a small capacity pack for qualifying & a large pack for the finals.
My car will just do 20 mins using 2200KV motor, 4S 5000mAh battery. The power of the battery is (P=V*I) 74W, 3.7W per minute of run time.
Hope that helps, a bit.