That is not an easy and straight forward answer. there are more than a dozen variables that one would need to have an answer. Variables such as; Vehicle weight, max speed, number and size of tires, type and number of motors, the use of vehicle, desired range, accessory requirements (lighting, controls, heat, A/C, power equipment, etc), not to mention the battery technology. I would think as one would gather the information the one would total the power requirements, then work on defining the battery (then the weight of the battery would have to be added to the equation and the re-calculations would begin anew). It a process and not a simple one. Good luck and may the forces be with you.
If you looking for high power density battery, consider to use Manganese or Phosphate or Nickel-based Lithium-ion. You can look in Max Discharge Current in the Li-ion cells spec. Sometimes it is expressed in 1C, 2C, 3C, etc. Higher xC means higher power density (of course it should compare with same battery capacity). But, it just about power density or specific power of the battery cells, not the car. Like Mike says, the specific power of the car depend on it net mechanical power and weight.