Unless you've got a motor problem, most upgraded guns don't draw more than 15A.
Anyway my 7.4v 1600mah LiPos are 20C (handles up to 32A continuous), and they're the size of a NiMH 6 cell mini battery.
Only place where you might have trouble is your motor. I've installed many triggermasters, and they just WILL NOT work with un-insulated systema magnum motors. Any motor that has a bent armature, short, or draws too much amperage, will constantly overheat your triggermaster, either on long bursts or excessive semi-auto.
That being said, if you have a GOOD motor, you'll have nothing but good times lol