I use inexpensive 6A 2-core mains cable for most jobs and is not so thick it is hard to solder into the connectors.
Bear in mind that the current rating of the wire is NOT the maximum current the wire can conduct without melting, but the maximum current recommended for power transmisson, and is v-v-v-ery conservative.
The current rating is most relevant for electrical installations, which have of course to conform to the regs.
The actual wire thickness required will depend on the current delivery of the amp, and can be calculated from the impedance of the cab and the maximum power using the formula W=I^2R.
So 6A into a 4ohm load equates to 144W.