On Screen Display
Minimum 150 pixels by 150 pixels for a screen at 72dpi.
Larger is better.
Minimum 1.2x1.2 inches (3cm) or larger.
If you print the QR code too small, it may become unreadable for some devices.
Know the Scanning Distance
The ultimate size depends on the user's scanning distance. As a rule of thumb, divide the scanning distance (in inches) by 10 to determine your print size. For example, if you expect users to scan from a distance of 3 feet = 36 inches = divided by 10 the QR code size should be 3.6 inches width and height (about 100 cm, so the code should be 10cm in width and height).