RE: what size battery pack
At full stall those servos may consume 2-3A of power each.
I just finished putting in a 6V 3000mAh pack for six digital servos on a small 25% ( 73" ) Yah 54.
I run a 2000mAh 4.8v ignition battery.
I had an 1800mAh battery for the servos and a 1200mAh 4.8v battery for the ignition, and I was NOT happy with the number of flights I could get safely out of those packs.
I fly 16-18 minutes minimum per flight, and 3-4 flights were enough to bring the latter packs down to the point I'd need to recharge them to fly safely again.
With the larger packs I can get a good 10+ flights in without re-charging the packs.
Dual 2500's as MetalicaJunkie suggests would be fine.