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.