Hi,
It depends how much current Your servos will draw, and how large margin You want to have if one of the LiPo packs fail in flight. And You probably do not want unnecessary weight so not too large LiPo packs.
Example:
ThunderPower ProLite+ 25C 2 Cell LiPo 480 mAh weight 30 grams, same but 730 mAh weight 39 grams. In Your case x 2.

I checked max amp draw from log (with help of Jeti Box) of my ESC Jeti/Hacker Master Mezon 90 with BEC and max amp was 1.7 amp. But that is max amp value, the avarage amp should be slightly less but to be on safe side one could use max amp draw when calculating how large LiPo packs one want to use (but I do not use separate LiPo packs for servos+receiver like You will have, I use instead Scorpion Backup Guard that is a 2 cell 500 mAh Lipo connected to receiver with special function to jump in if voltage is 5 volt or below like if BEC fail or similar). I have my BEC set to 7.4 volt so servos and receiver get that voltage.


I use these High Voltage (HV) Futaba servos:

Rudder: BLS171SV
http://www.gpdealera.com/cgi-bin/wga...pgm?I=FUTM0135
Elevators: BLS173SV x2
http://www.gpdealera.com/cgi-bin/wga...pgm?I=FUTM0137
Ailerons: BLS174SV x2
http://www.gpdealera.com/cgi-bin/wga...pgm?I=FUTM0138

/Bo