The Wirecutter is an excellent website that tells you what the best product is in any given category. Whenever I want to buy something in a category I don't know much about, I go to the Wirecutter.
Unfortunately, when I went to buy my wife a portable charging devices last week, they didn't have a review up.
They recommend the Satechi 10000 mAh model. For the same price as the one I got my wife it has 6,000 mAh. On the other hand, I don't think that looks like it would fit easily in my pants pocket at 5.7 inches long.
It's a trade-off: portability vs capacity.