Hello,
The battery pack minimum capacity is directly related to its discharge rating. You have to find out your motor max consumption first, and then select a battery.
Example: if your motor consumes 35A at full power (full charge battery), then a battery that takes that load would suffice (Ex 3500 mAh battery with 10C disch rate = 35 A)
Different brand and model batteries normally have different maximum discharge rates, known as "C rates", clearly printed on the outside of the pack.
For 35A:
a 3500mAh battery with 10C discharge rate (3500x10 = 35000mAh = 35A, the same the motor pulls)
a 2000mAh battery with 20C (2000x20 =40)
etc...would be enough.
I recommend going for over rated discharge packs, packs that would give you at least 20 % more discharge rate capacity than you motor consumption. Keeps your batteries cooler, safer and increases their service life.
Fly safe, be safe