Match the BTU rating to the room size: 5,000-6,000 BTUs for up to 150 sq. ft., 6,000-8,000 BTUs for 150-300 sq. ft., 8,000-10,000 BTUs for 300-450 sq. ft., 10,000-12,000 BTUs for 450-550 sq. ft., and 12,000-14,000 BTUs for 550-700 sq. ft. Adjust for ceiling height, sunlight, and usage.