Properly sizing an air conditioner is important, especially on whole-house systems. An improperly sized unit can cost you more to operate, and worse, leave you either uncomfortably warm, or cold and clammy depending on whether it is under or oversized. To shop knowledgeably, you should know that air conditioners are rated by how many BTUs of heat they remove in an hour. Whole-house air conditioners and heat pumps usually also list cooling capacity in tons, where one ton removes 12,000 BTUs per hour. Therefore you may hear people talk about a 2-ton air conditioner. That would be one with a capacity to remove 24,000 BTUs per hour.
It is recommended to get help on a major purchase like this from a professional because simple guidelines and rules of thumb may or may not work. Some contractors joke about the rule-of-thumb that if you can completely cover the house with one finger on your extended hand when standing at the street in front if it, it needs a one ton unit, but if it takes two fingers, that takes a 2-ton unit. That's not good enough, and if it's your money and your comfort, you want to be sure it's properly sized.
Undersized systems will run continuously and never get the house down to the thermostat setting. Oversized systems will cycle on and off frequently and do not adequately remove humidity, leaving occupants feeling cold and clammy.
Sizing heat pumps has its own special challenges. In warm climates, where they are being sized for a cooling load, they may be oversized for the heating season, and then the reverse is true. A heat pump in a colder climate sized for heating may be oversized for cooling. These considerations make it all the more important to rely on the expertise of a professional when it comes to sizing a heat pump for your home.