Coming from a radio control racing background, the more C rating the better
People have a misconception that Volts is what gives power when in fact it is the Amps
You should be able to fit a 2000mah or 2amp Lipo into the buffer tube (or crane stock)
The last CQB game I played, I used a small 1300mah Lipo that lasted me all day, both cells were tested at (on game day) at 4.18v each, when I got home & tested again, this had gone down to 3.97v each cell so not much was used
Most airsoft motors if not spammed will not draw many amps