I would recommend two 2300mah A123's though two switches for the RX and a 1100mah A123 for ignition if it can handle 6.6 Volts. The only sizes of A123's are 2300mah and 1100 mah. If it's not that size it's not a A123 it's a off-brand LiFe. I used a single 2300 on the RX of a 50cc plane and was pushing it to get 4 flights. The amount of Mah's you use depends on the plane, servos, flight length, and your flying style. I prefer to use a Smart-Fly Optic kill and no switch for the ignition.

The place I prefer to get my batteries is Hang Time Hobbies. http://www.hangtimes.com/ Even if you don't buy your batteries from them it is well worth your time to read through the information on their website.