I really like Lumsing's Powerbank for its fast charging 5v/2.1amp USB slot. Just yesterday, Pesco was struggling with a slow charging battery and asked for one with a faster charging port! Here it is.
This harmonica sized and shaped battery is about 5.5" long and 2.5" wide. The Power Bank easily fits into a jacket pocket and won't be a burden to carry around. At 10400 mAh you should get one and a half charges of an iPad or many of smaller devices.