What should be the mimimum print size for QR Code?

Usually, the minimum printable size for the QR code is 12mm x 12mm, with a preferred 76 dpi. The part from the top two eye border should be considered as size, as shown in the pic below.

However, depending on your Business Card size, you may make it 20 x 20mm or more. Try not to go below 12 x 12mm.

However, for any print size, please make sure to test the sample first with default iOS and Android scanners before doing the mass printing.