The pi can't draw more than 1A due to a fuse stopping the board drawing more than that. So how big a battery you need really depends on how long you want it to run. If you have a 500mAh battery, that will output 500mA (0.5A) for one hour, or 1000mA (1A) for half an hour. Similarly, a 2000mAh battery will give you 2000mA (2A) for one hour or 1000mA (1A for two hours). So really it's up to you, your battery, and your budget.