Your local HVAC professional will be able to provide the most accurate recommendation, but you can do a rough calculation using the square footage of the space you want to heat and cool. As a general guide, 100–200 sq ft usually needs 9,000–12,000 BTU, 200–450 sq ft needs 12,000–18,000 BTU, and spaces up to about 800 sq ft may need 24,000 BTU. It’s often best to size slightly higher, especially if the room gets a lot of sun or isn’t well insulated, since a unit that’s too small will struggle to keep up, while one that’s too large can run inefficiently.