There is no single perfect print size. The right size depends on viewing distance, the amount of data in the code, the printing method, contrast, and the surface where it appears.
Start with viewing distance
A code on a business card is scanned from close range. A code on a wall sign or vehicle graphic must work from farther away. Increase the code size as the expected scanning distance grows.
Protect the quiet zone
Leave clear space around every side of the QR code. Text, borders, patterns, and images placed too close to the modules can make the boundary harder for a scanner to identify.
Prefer vector artwork for production
SVG artwork stays sharp at different sizes and is usually the best starting point for professional layouts. PNG can also work when it has enough resolution at the final placed size.
Do not trust an on-screen test alone
Paper stock, ink spread, glare, folds, curved packaging, and installation conditions can all change scan performance. Test a physical proof using the real materials and final dimensions.