I have used these three methods to determine the battery capacity:
1) When you stop the car after some lengthy drive, check the drive data (CAR button -> Selection -> drive data), note the trip miles and the energy economy. Note also the remaining range (screen to the left of the speedometer). Add trip miles and remaining range. Divide the sum by the energy economy. E.g., trip was: 37 miles, remaining range is: 47 miles, energy economy is: 4.4 mi/kWh
(37mi + 47mi) / (4.4mi/kWh) = 19kWh
2) When you stop the car after some lengthy drive, check the drive data (CAR button -> Selection -> drive data), note the trip miles and the energy economy. Read the battery level (below speedometer). The battery level has 16 tick marks from empty to full, record the "battery capacity used" (not the "remaining capacity"). E.g., the trip was: 34 mile, energy economy is: 4.1 mi/kWh, battery capacity used is: 7/16
34mi / ( 7/16 * 4.1mi/kWh) = 19kWh
3) Drive your car until the battery is almost empty and charge it. My JuiceBox app on my phone tells me next morning that it charged the car with 18.4kWh.
I understand that there is also the possibility to use an OBD device (like OBDeleven) that can read the battery capacity directly from the car, but I have never used this.
cheers,
Werner