If you get packs made out of AA cells or larger (for standard servos) you should be fine. The higher the capacity, the more flights you'll get out of it. ALWAYS check your batt pack voltage before a flight, either with a loaded voltmeter or an on-board voltage monitor. In general, I use 4-AA cell packs (1000mAh+) for trainers or scale planes with standard servos, 4-A cell packs (2000mAh+) for my Aerobatic/3D planes with standard servos, and 5-A cell (2000mAh+) packs for my Aerobatic/3D planes with high torque servos. Guys with Giant scale planes seem to go with sub-C cells or LiPo cells with regulators.