Typically, you get wire size from an ampacity table. (Google "ampacity table").
There are two main things to consider. First what is the normal current? Wire must be sized adequately to carry the normal current without excessive voltage drop. Usually in short wires this is not a problem unless the discharge rate is very high. But you can look up the resistance of the wire (lots of charts online), per length, and calculate the resistance of your wire harness. Then multiply resistance by current to get voltage drop. Often, I choose to allow 0.05 to 0.1V of drop in the wire, but it depends on many details.
Second thing is fault current. What is the highest fault current that could possibly flow for an extended period? For example, if you have a 1A fuse inside the battery pack, it can pass 1A for an extended period. It would be a good idea to choose a wire that will not overheat with, say, 1.5A. Then, no matter what fault occurs on your load, at least the wire insulation will not turn black or start a fire.
If the battery does not incorporate a fuse or other over-current protection, you should use a different battery that does. Lithium ion battery packs should have a built-in protection circuit that limits charge and discharge current and voltage. This is widely known in industry, so there is a good chance that the pack you are considering does have a protection circuit built-in to it.