Why is this a problem? Because a key element of feeling "comfortably cool" is removing humidity from the air as well as reducing the room air temperature. While an over-sized unit may quickly reduce air temperature, it will not properly reduce humidity or run efficiently. Oversizing results in humidity discomfort and expensive operation.
To properly size a room air conditioner, I suggest using this sizing chart provided by the Energy Star® program of the DOE (Department of Energy).
