-small
-through charge compatible (can be charged and charge something at the same time)
-mini usb or other solid charge input (not micro like android phones as this get knocked out to easily)
I have one of these: https://www.powertraveller.com/en/shop/portable-chargers/outdoor-adventure/powermonkey-explorer-2/
It comes with micro USB in and out as standard, but you can buy a replacement cable for mini USB, which I have. Alternatively, you can use the standard cable and adapter to plug a mini USB cable into that.
Only downside is it’s not ‘small’. It’s not huge but it’s not one of those tiny pen sized ones.
Other pros: through charging works, the cables seem to be pretty robust, waterproof, shockproof etc.