[Sebb767]

That's dependent on the zoom factor and focus distance of your lens and therefore really hard to say in general.

In theory, you could even make a micrometer-sized QR code and scan it using a phone-microscope-attachment - so if you're really trying to push it, probably smaller than you can reasonably print/manufacture.

For a more practical usage, when we worked with QR-Codes, about 3cm by 3cm (or 1.2 inch) seemed to be the smallest unit not causing trouble; smaller than that and problems with focus started appearing for some clients and scan times increased. Of course, that's only anecdotal evidence.