A QR only needs to be as large as the scanning distance allows. The simple rule: the code should be at least 1/10 of the distance from the scanner. From a metre away, that is about 10 cm; from arm’s length, 2–3 cm is plenty.
Leave a blank margin around the code equal to four modules (about 10% of the width on each side). Without it, cameras struggle to find the edges. QRVolt adds this automatically.
Use level H when you add a logo or print on textured stock — it reserves recovery data for damage. Keep dark modules on a light background and avoid rounding that smears edges. Always scan-test the final print; QRVolt verifies scannability before you export.