How do I choose the right BTU for my room?

As a general rule: 5,000-8,000 BTU for rooms up to 150 sq ft, 9,000-12,000 BTU for 150-500 sq ft, 18,000 BTU for 500-750 sq ft, 24,000 BTU for 750-1,000 sq ft, and 36,000+ BTU for spaces over 1,000 sq ft. Consider ceiling height, insulation, sun exposure, and climate for accuracy.