The size of your house is a major factor that determines the size of the air conditioner you need to install. However, other factors, such as the five discussed below, also matter.
The number, size, and orientation of your windows determine the amount of heating or cooling needed during your air conditioner installation. This is because the windows transmit more heat, in any direction than solid parts of the walls. Therefore, a house with more or bigger windows than the other requires a bigger AC; assuming other factors are constant. The orientation of the windows also matters; for example, having more sun facing windows means more heat enters the house, which also calls for a bigger AC.
The more your house is shaded, the less it needs air conditioning. Therefore, the presence or absence of shade also determines the size of the AC you should install. For example, if your house is surrounded by tall trees, buildings or other structures, then you won't need as big of an AC as you would need if those things didn't throw shade on your house.
The number of people who live in your house is also a factor in the calculation. The more people there are in the house the bigger the AC you will need. This is because all bodies emit and absorb heat, and everybody in the house will need to breathe in conditioned air. Therefore, increasing the number of your house's occupants increases the AC's load.
Heat Generating Activities
Activities that generate heat will necessitate more cooling, which means a big AC. Examples of heat-generating appliances include large kitchen appliances, power tools, and electronics, among others. Therefore, if you have a home workshop that involves engines or power tools, you will need a bigger AC to compensate for the heat generated by the workshop.
Lastly, the state of insulation of your house will also be considered. The state and amount of insulation within the walls and attic determine the amount of heat lost or preserved within the house.
AC technicians have a professional way of determining the size of AC needed for a given house; you won't have to do the calculation yourself. However, it pays to know the above factors so that you can understand why the technician is suggesting a given AC size. That way you will not have a problem when the installers suggest a bigger or smaller AC than the one in your neighbor's house.