When it comes to spare phone chargers, the smaller the better, and you'll have a hard time finding one much smaller than Anker's PowerPort III Nano. The little fella packs 18 watts of charging power into a pocketable package, and you can grab one today on Amazon for just 12 bucks.
The charger probably offers the same amount of juice as the one that came with your phone (if not more) in a markedly smaller footprint. It doesn't come with a USB cable and the prongs don't fold, but still, it's a heck of a deal on a pretty great little charger. Head over to Amazon to get one.