the size of the battery depends on the size of the heli. 300 size electrics typcically use between 1000 to 1350mah, 450 use 1500 to 2500mah. both 300 and 450 use 3S1P 11.1v batteries.

when you move to larger electrics like the 500 to 600 class the size of the battery depends on the size of the motor, some use 4S, 5S or 6S batteries. some can use two 3S in series to get 6S. The 6S batteries can be up to 6000mah and cost $300.

so, what size heli are you talking about?