If too small the reserve run time and cranking power will be limited, could cause premature alternator failure as it would have to work harder to bring the battery back to full charge cause it would take more power from it to start the engine, may not fit the battery box so it could come loose and short out on something. If it's too large it will not fit the battery box and the odds of shorting out are great, but you will have plenty of cranking and reserve power.
Are you asking about the physical size of the battery or the electrical capacity of it?
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