What is a good capacity for a power bank?

If you are looking for Power Bank for Smart Phone , Tablet You should choose a power Bank with Capacity of 10000 mAh to 20000 mAh. As you know smartphone, tablet itself comes with normally 5000 mAh battery, So once your Smartphone or Tablet discharge you can charge atleast 2 times. Buy Best Powerbank for your Smartphone and Tablet from Here - Link 

and if you are looking for Powebank for Basic phone then you can choose Powerbank with capacity of 2000 mAh to 5000 mAh . You can buy Top Selling Powerbanks for your mobile from here Link