RE: Is there a plain simple answer Li-Poly
Often, the cell size is dropped now because of the incredible advancements in battery technology over the past few years.
It is easier to speak in general terms if you don't actually measure the current in your setups. For example, in a Speed 400 geared setup, we can assume a ballpark current limit of around 10amps. For the geared Speed 300 or 350 setups, it is about 6amps. This gives you an idea of the current needed to be delivered by the battery whether it is NiCd, NiMH, or Lithium technology.
As for cell count, a 6-7 cell NiCd or NiMH setup translates to a 2-cell Lithium pack. Often, on 8-cell NiCd setups, we option for a 3-cell Lithium pack and assume the use of throttle management will keep our motors safe for a reasonable longevity. The extra full-throttle power is usually only needed on take-offs or loops, etc. A 3-cell Lithium pack is equivalent to a 10-cell NiCd or NiMH pack.
If all else fails, usually the battery type gives you a hint of the size. If you are talking about a 600mAh NiCd battery meant for a Speed 400 power system, this often hints toward a 2/3A size.