RE: HPI Mini Recon
You need to stay with approx same voltage you use with brushed motor, which should be 2S voltage. As to mah, max size that will fit is the right answer. You want as much capacity as possible. Measure carefully and millimeters count. note that you do NOTwand to smush the lipo in to fit. Setting up for a rather spectacular fire that way.
I also switch to deans connectors sicne i use them on my planes, but that is purely optional.