Guidance on Speaker Wire

That is a very long run......

1) Choose a wire at most 8 gauge, not any more.  Basically the idea is to reduce the resistance.  The thicker the wire (lower the gauge), the less its resistance.  The resistive contribution of the wire should be no more than about 5-10% of the output impedance of your amplifier, to minimize losses.

2) Make sure it is copper as pure as it can be.  Silver is a bit better but at that length it will cost a ridiculous amount.

3) Do NOT coil the wire, keep a straight line run as much as possible, otherwise inductance will increase.

4) Try and change your room as a last resort, that long a wire is very unusual.  I am sure your speakers are not 120 feet apart....

#4 or #6 welding cable would be a good and inexpensive choice 

I use Mogami speaker cables exclusively.  They are used in studios and should accommodate longer runs.

check with Auditorium23 in Germany, their cable is designed for minimum resistance and very high quality. Not sure though whether they will ship that length. Better solution would be having the amps behind the speakers on balanced interconnects

+1 @elliottbnewcombjr 
For better performance, it is much better having long interconnect cables (balanced if possible) rather than long speaker cables. Generally, you should try to keep speakers cables as short as possible