Battery

[Cat966c]

Hey all, I'm at the dry fitting stage of my loader build, and I was wondering what kind of battery do I use? I want an 11.1 volt, because I want full power from my pump (it's a 12 volt).
However; I'm worried about the size of the 4000-5000 batteries because space is an issue. I'd like to use the 5000 mah ones because that's what's in the rc4wd loader, but they are pretty big. I'd like to be able to use a smaller sized 11.1 like a 1000 or 2200. Would those be powerful enough? Also What does the mah have to do with it? What is it?
Thanks 🙂