The unit size can usually be determined by the total sq. ft. area & nature of the environment, pollutants being addressed, as well as airflow conditions within the environment. After you complete the free test for indoor air pollution in your home, a purifier will be recommended to you, based on your environment. As a rule of thumb, it is usually best to start with more equipment than necessary and adjust accordingly to the needs of your environment, versus adding equipment at a later time. This way it's more cost-effective.