I use this Super Senior (Nitro Planes) set up for exclusively for primary training with an OS .35AX and the servos are all Hitec 225 Mighty Minis.
If you are flying 72mhz then a four cell (4.8 volt) pack of 700mah's or better is sufficient.
If you are flying 2.4 then a five cell (6 volt) pack of 1650mah's or better is sufficient. These are what I reccomend and consider the minimums for the on board battery supplies.
You will enjoy your Senior, There has been no better trainer ever.
John