Assuming a standard business card size of 9 x 5cm (3.5 x 2in), the QR code must be around 2 x 2cm (0.8 x 0.8in) to ensure it is scannable. You can make the code bigger to meet your design needs; however, it is not recommended to make it smaller.

A good rule of thumb is to make the business card QR code large enough for smartphones to scan but not too big to overcrowd the card's design. The placement of the code is also important: placing it in a difficult-to-spot area might result in fewer scans.

If you plan to make an unconventional business card with a unique design and size, follow the best practices for QR code sizing to ensure the code is easily scannable after printing.