Yeah, I second the "get a balancer" thing.

As for what pack, with any battery type, the right pack size is determined by a number of factors.

- how big is the battery compartment? Really, no sense in buying a pack that won't fit in the plane.

- number of cells, if they recommend 3 lipos, then you might want to stick with that. With lipos, there's a big difference between 3 and 4 cells. With Nicad/Nimh cells, you could play with cell count more, since it was only a 1.2v difference. In your case a 4 cell pack might not fit, and will probably burn up the motor and/or ESC. A 2 cell lipo won't burn anything up, but probably won't have the performance you want.

- capacity has 2 issues. One is flight time. More is better. The other is current capability. A 10C pack that is 2100mah can deliver 21amps. A 10C pack that is 1500mah can only deliver 15amps. So you need a pack big enough that you won't trash it. And the less "C" you draw, the longer the pack will live.

So, for flight time and battery lifetime, you should generally cram the largest capacity pack you can manage for the weight and battery comparment space. Once you've been doing electrics for a while, and want to do things like minimize weight, you can try smaller packs and limiting the current you pull and such. But for sport flyers, bigger packs are the way to go.

Oh, a couple hundred mah on a 2000ish pack isn't noticeable, so a 2100 and a 2200 is about the same for real-world sport flyers. Anything in that range is good.

Also, be aware that some cells are shorter and thicker, others are wide and long and flat and so on. So watch all 3 dimensions of the battery compartment and make sure your pack will fit if you go with something besides the recommended pack. There are some battery vendors out there that acutally have "pack builder" web pages that allow you to enter a battery compartment's dimensions, and they will run the numbers for you.