To determine the right size AC for your home, consider factors like square footage, ceiling height, insulation quality, number of windows, and local climate. A professional can perform a Manual J load calculation for precise sizing. As a rule of thumb, you need about 20 BTUs per square foot of living space. However, oversizing or undersizing can lead to inefficiency and discomfort. It's best to consult with a licensed HVAC contractor who can assess your specific needs and recommend the most suitable AC unit for your home.