A practical starting rule
For a typical phone camera, make the QR code roughly one-tenth of the expected scanning distance. A code scanned from 10 feet away should be approximately 1 foot wide. Treat this as a starting point and always test the final design.
Keep the quiet zone
The empty border around a QR code helps a phone distinguish the code from nearby text and graphics. Do not crop the border or place other design elements inside it.
Contrast matters more than decoration
Use a dark QR pattern on a light, solid background. Avoid gradients, reflective materials, low-contrast colors and busy photographs behind the code. If you add a logo, use the highest error-correction setting and keep the logo relatively small.
Test the actual finished piece
A code that scans on a monitor may behave differently when printed. Test at the intended size, material, lighting and distance with multiple phones before ordering a large print run.