The minimum size of a QR code for it to be read correctly is 0,4" or 1cm.
Here is a formula that will help you choose the best size:
Scanning distance / 10 = Minimum QR code size
(as long as this number is not lower than 0,4" or 1cm)
For example, if your visitors will be 40" or 100cm away, your QR code's size should be 4" or 10cm.
NOTE: Always remember to test your QR code before printing it.