A battery should be big enough to allow reliable cold starting. The standard recommendation is a battery
with at least one Cold Cranking Amp (CCA) for every cubic inch of engine displacement (two for diesels). CCA rating is an indication of a battery's ability to deliver a sustained amp output at a specified temperature.
Specifically, it is how many amps a new, fully-charged battery can deliver at 0 degrees F for 30 seconds and still maintain a minimum voltage of 1.2 volts per cell.
A rule of thumb says a vehicle's battery should have a CCA rating equal to or greater than engine displacement in cubic inches. A battery with a 280 CCA rating would be more than adequate for a 135 cubic inch four-cylinder engine, but not big enough for a 350 cubic inch V-8.
Battery manufacturers have been trying to outdo one another by introducing batteries with higher and higher cold cranking amp ratings. There was a time when a battery with a 550 CCA rating was considered a powerful battery. Now there are batteries with 650, 750, 850, and even up to 1,000 CCA available.
One reason for the "amp wars" between battery manufacturers is that bigger is definitely better. How much overkill is really necessary to assure reliable cold weather starting? Two amps per cubic inch of engine displacement? Three, four or five amps? The bottom line is bigger sells better.
The difference between a group 23 battery and a group 24 battery is 1/2" in length, 1/16" in width and 7/16" in height. It does not sound like much, but it is enough of a difference that the longer battery might not fit the space provided for the shorter battery if a swap were attempted.
Since there is little or no effort on the part of vehicle manufacturers to standardize original equipment battery dimensions, aftermarket battery suppliers are faced with the task of trying to cram as many amps as they can into the smallest battery case that will fit the most applications.
Consolidation reduces the number of different batteries a jobber has to stock to cover the various vehicle applications. It also simplifies manufacturing by building fewer basic battery sizes.
The most powerful battery in the world will not be able to do its job properly if battery cables are not up to the job. One often overlooked source of cranking trouble is undersized battery cables. If the original equipment cables have been replaced with cheap ones with undersized wires, the cables may not be able to deliver the battery's full amp load to the starter.