Short answer would be that 12 gauge will almost certainly work well. Long winded answer is that there are numerous tables that deal with "minimum" acceptable wire gauge for a given run length and ohm load. Unless you're running low ohms for very, very long runs 12 gauge should work great.
SPEAKERS: 3 x DIYSG 1099's LCR; DIYSG HTM 6's and 8's surround; 4 x RSL C34E's atmos.
SUBS: 4 x UM18-22 Mini Marty's and 2 x 21DS114-4 Devastators
SIGNAL CHAIN: Denon X4400; Minidsp 2x4 HD; 2 x Jensen Iso Sub 2RX; 3 x NX6000d.