Size Does Matter but There's More To Consider when Purchasing a Home Air Conditioner

One major factor is the room's position in reference to the heat of the mid-day sun. Rooms on the south side of a home are in direct sunlight most of the day. Ol' Sol's rays heat up a room more than a northern exposed room, a lot more. That's the case even when shades and blinds are drawn. The heat of the sun does penetrate the walls of even some of the best insulated homes.  That extra heat is makes the cool off period of time take much longer in a room where the AC may be shut off for the day and only used at night as in a bedroom. If the room's A/C is continuously used it during the day has to work harder when there is a hot sun toasting the outside walls and can use a lot more energy rather than turning it on the eight hours of so a person is in the room.

It's a shame to have to pay for extra energy when purchasing the proper sized unit in the first place would make one more comfortable and save dollars in the long run.

Ceiling heights are sometimes taken into account on box top stats on A/C units, but not always. A ceiling higher than the average 8' has more air to be cooled in it therefor more BTUs may be needed. 

Another factor can be the number of people in a room. Each person gives off heat so an additional 500 to 700 btu's per person over one could/should be factored in.

Any room that is close to a doorway that is opened often needs an A/C that can keep up with the change of air temperature and humidity that comes in and lets cooler air escape. Kitchens also need more BTUs because of heat from burners and ovens. Exhaust fans also pump out much air that must be replaced.

Energy efficient lighbulbs also help keep a room cooler than incandecent bulbs. Any room with many non-efficent bulbs may need a little bit more BTU power, especially if it also housed a PC that emits heat as do other appliances like refrigerators.

It's better to have an A/C that's just a bit too much for a room rather than one that cannot keep up with the heat and humidity leaving you in a hot uncomfortable situation.