What's the smallest possible size for printing a QR code?

The easy answer is 15mm x 15mm.

The slightly longer answer would be that you want to make the QR code as large as you can. You (generally) want users to be able to scan the QR code quickly and easily - even with an old scratched up phone camera. Making the QR code large and clear will make it work better.

The complicated answer is that it depends. QR codes are made of lots of small squares (or dots if you have styled your QR code). The number of dots in a QR Code will depend on the length of the data that you are storing in the code. So a short URL will result in a QR Code with less squares than a long URL.

Additionally, QR codes have error protection built in. The higher the error correction, the more squares you are likely to have.

In reality, the readability of a QR code is closely related to the size not of the whole QR code but the small squares inside it. If these are too small and blur/merge into each other, then the QR code will become difficult or even impossible to scan.

So. Keep your URL link as short as you can will help you make your QR code small as well, A short URL could allow a QR code down to perhaps 1cm square. A long URL might required a 2cm or larger QR Code.
Seritag - 19 Jan 2023