Oh, by the way, Innokin makes other power sources. You can get this for about $45 online:
Here's a pic of the bottom:
It does everything that the cylindrical ones do but it's a more robust battery and you can use the USB 2.0 port on the bottom to recharge your cell phone or whatnot.
Here's a random internet weirdo holding one so you can get an idea of the size:
