If we're talking theory, the pixel size doesn't matter - it's the amount of information encoded in the QR code that matters (ie. excluding ECC bits!). In turn

If we're talking theory, the pixel size doesn't matter - it's the amount of information encoded in the QR code that matters (ie. excluding ECC bits!). In turn, that means it is theoretically possible to decode a QR code where each 'pixel' of the code is smaller than the pixel of the camera sensor observing the code. While it is theoretically possible, nobody has yet done so...