G'day Mate,
800mah is fine for a trainer, 2 guys in out club have 800mah in their trainers with the DX7, & they get enough flights in a day to take them past the overload point, meaning they have had enough.
I use the DX7 in 3 planes so far, soon to be 4, I use 1100mah, in 2 of them, & 2 battery packs in the larger one, it has a 160FX in it.
I doubt you would need more than 1500Mah 4.8 volts in you trainer, after all you are really only using 2 servos, that have flight loads on them, while you are flying, ailerons & elevators, you rarely use the rudder, & the throttle has no flight load, so stick with the KIS principle, keep it simple, & you will be fine.
But by all means buy yourself & battery tester, like this one, http://www.rojshobbies.com/Radios.html
Scroll to bottom of page.
They test your RX pack, with a load, to make sure you have enough battery capacity to keep flying.