The size: this is not about how well it fits the pocket. It is about the power that can be carried within. Many of the options available usually has a number at the front that lets you know about mAh. This means milliamp-hour. When the number is higher, it means there is more power stored within and mire recharges doe the smartphone. Bigger mAh is also ideal for the bigger gadgets.
Power output: you need to consider how fast you will be able to charge your gadget using the alternative charging. It is important to be able to charge the phone/tablet as fast as possible. Your standard USB port on the component can fit the standard cable. However, it is important to know that the transmission can vary a great deal, depending on what you use.