You shouldn't have a problem with the batteries or charging that far apart.

As to size, it depends on how you fish. A single 24 would give you 120-134 minute RC. A single 27 would give you about 160-180 minute RC. A single 29 or 31 gives you about 205 minute RC. A pair of 24s would give about 240-270 minute RC. A pair of 27s would give about 320-360 minute RC. A pair of 29s would give you about 410 minutes.