A good battery accepts and holds a charge. It can produce amperage close to its rated output and just needs to be recharged. While a bad one will not hold or accept charge leave along coming close to the required amperage. You just have to replace it.
A car battery life is about 3 to 5 years, so going by its life, you will need a new battery.
A low or dead battery does not mean your battery has failed, or that it needs to be replaced. A good battery can run down for any number of reasons: somebody might have left the lights on, you have not driven enough to keep the battery fully charged. It could also be that the car has been idle with lights and music system on with engine off, that means battery was drained without getting recharged from the alternator system that works with the engine rpm. Or there is an electrical problem which is draining power from the battery while car is off: like anti-theft system, music system, bluetooth and internet connectivity.
Many auto parts stores will test your battery for free. On your own, try the load tester. If the battery voltage drops below 9.6 volts during the test, the battery is bad. If it gives a reading of 0 volts, your battery just had a short circuit. If the battery stops reaching 10.5 volts, it has a dead cell. If the battery is fully charged and the voltage is 12.4 or less, the battery is sulfated.
You could also use an electronic ‘conductance’ tester. It sends an alternating frequency signal through the battery to determine the condition of the cell plates inside the battery. Such tester gives an accurate reading.
As for the size, the stock-size battery should be ideal. In GCC conditions, a smaller battery may not fail either. A larger battery has a lot of extras - expense, toxins, weight. Increasing battery size might allow running your car, but it is not an advisable upgrade unless you really require that additional amperage to power up some special device like heavy music system with amplifier and woofer, have additional headrests DVD screens that chew lot of power and slight increase in battery output will ease the load. Upgrade the battery to max 10-15% increment only, like if OEM battery had 60 AMP so you can safely upgrade to 65-70 AMP but not higher than that without consulting proper electrician.
If you need more help, or need specific answer for any question then try the ‘Carnity Forum’ (http://carnity.com/) section or find car businesses near you with the Carnity ‘Business listing’ (http://carnity.com/business_listing) section.