When purchasing a new system, it's very important that a licensed dealer visit your home to perform a complete evaluation, or "load calculation." By doing this, he will look at all the factors that affect the heat gain and loss of your home.
The dealer will take into account the climate you live in, as well as the size, shape, and orientation of your home; it's not enough to merely calculate square footage, although he will do that, too. What type of insulation and how much you have is important along with window size, type, quantity, and the directions they face. He might measure walls and floors; he'll see what materials the house is made of. He'll check seals, estimate air leakage, and locate existing vents and ductwork.
By combining these and other factors, the dealer will evaluate what size unit your home requires. However, if you are replacing an existing system, you can give your dealer a starting point. You can provide to him the model and serial numbers of your existing units, found on the rating plate.
But you will still require a load calculation. An over or under sized unit is one of the most common problems with any system; when a licensed dealer performs a load calculation, you will have a more efficient system and a more comfortable home.