What type of amp will be powering this system? For this long of a run you really need to use 70 volt line transformers.

If fidelity is of no concern, any wire can be used. The resistive losses in smaller guage wire will only limit the amout of power that reaches the load (speaker). For example, if the speaker is 8 ohms and the wire has 8 ohms of resistance (unlikely) the speaker will receive half of the power. The other half will be lost in the wire. Bigger wire=less resistance=less loss.

I hope you plan on using horns. Any direct radiating speaker would not be efficient enough for this application.

A "Cobraflex' is a very good speaker for this.

Regarding the wire... bigger the better. You could use 12/2 house wire.