We are discussing air conditioning here folks, and bigger is not better. Oversizing an air conditioner will create damp chilly air and leave more cold air on the lower levels of your house. You want to size the unit according to the heat gain calculated on the building. This is a process to determine how much outside heat infiltrates your house and how much cooling you need to remove that heat as it accumulates. If you have a two storey house the second floor will always be warmer than the main floor and the basement. The reason for this is that cold air drops like water and your furnace fan has the job of pushing against gravity to the top of the house where most of the heat is generated. Add a second disadvantage with the thermostat on the first level where cool air will want to accumulate telling your system it has done its job. Often people will experience up to a five degree difference between levels.
I tell my customers it is better to size the air conditioner a little smaller so that the system will run longer pushing more cold air upstairs before the thermostat is satisfied. It is your best bet. It does not hurt the outdoor unit (the condenser) to run longer and it actually uses less energy (it is when the compressor starts that the lights dim). If you are attempting to cool a third floor with the furnace in the basement your chances are slim. We have been installing numerous ductless units for 3rd floor applications with excellent results. Ductless units are an excellent way to cool rooms and areas that your central air conditioning cannot reach.
Call and ask Alan or Fred for more information.