The size of an air handler is related to the size of the condensing unit. The air handler is a fan blower, that blows the cold or hot air from the condenser unit outside or to the rest of the house. The air handler is what handles the temperature of the cooling/heating system. Most air handlers are located close to the thermostat; and must have their filters cleaned to stay in optimal condition.
Size the air condenser unit. Have an engineer do a load calculation for your house. This will let you know how many tons per square feet your air condenser needs to be. On average you will need one ton of capacity per 500 square feet.
Size your air handler. Size your air handler unit based on the size of your air condenser unit. If your air condenser unit needs to be four tons so must your air handler. You can put a larger air handler by one ton more, but for optimal performance they should match.
Check electrical capacity. Depending on the energy requirements your air handler will use, you will need to make sure your current electrical setup can support it. If your electrical capacity is not enough you must redo the electrical connections; or you must size your air handler and air condenser to the maximum size supported by your electrical connections. To check your electrical capacity you will need to view the circuit breaker panel for the air handler and see the maximum rating on it, compare it to the air handlers electrical requirements found in the manual.