Rule: distance ÷ 10 = QR width.
- Business card (30 cm scan): ≥ 2×2 cm
- Counter sign (1 m scan): ≥ 5×5 cm
- Restaurant menu (50 cm): ≥ 3×3 cm
- Outdoor poster (5 m): ≥ 50×50 cm
- Billboard (20 m): ≥ 200×200 cm
Test from actual viewing distance before printing.