I've been doing some research for ultra-portable batteries. What does this mean? It should be convenient to carry by being very small (pocketable) and also having a built-in micro USB plug or cable. This prevents the need to carry an additional charging cable too. Since these are tiny, they are not suitable for full or multiple charge cycles. Rather, they are to be used as a last resort for a quick 30-50% (or more) battery. They won't have a built-in AC wall adapter either. Finally, if you don't mind carrying a USB cable then I strongly suggest you choose another, more reliable, brand's charger. There are many-MANY-many better options that these which are often sold for less money at higher capacities. Small form-factors are pricey due to their convenience and tech-limitations.
Here's some of my findings below. Again, do your own research if you do choose to buy one. I can't make any recommendations as I haven't finalized my decision yet. Some of these may be unreliable or unproven brands. Always check reviews and customer comments for further insight.
Smallest: Credit Card Shape or Similar:
Rectangles (slightly bigger)
Other
In my opinion they look hideous and take up way too much space (but you may think otherwise). My solution is to have an external battery with me and just plug it in whenever needed. Anker Slim2 will charge the phone one and a half times, is smaller than the phone, and basically the same thickness. Or for more power there's the Slim3 which should give you at least 2 full charges while being slightly thicker than the phone. Built in cable makes it very clean and easy to carry around. Plug it in and put it in your pocket or something.