I always carry one because running out of battery when you're out at night is pretty scary. I have this one by Anker and it gets me about 2 full charges before I have to recharge it. (On sale for $9.99 now, actually! That's a excellent deal.) Basically, I only need to charge it like... once a month because I usually just use it to keep my phone from dying, not try to get a full charge.
There are three different sizes - S, M, and L. I got the M (the one for Samsung Gear VR).
As for battery pack, I don't have mine yet. But I have this one on order, and should receive it tomorrow: https://www.amazon.com/gp/product/B005NF5NTK. I expect that it should easily fit in the case with everything else.