QR codes that actually get scanned
Size: at least 2 cm square for a counter card, bigger for posters — the rule of thumb is scanning distance ÷ 10. Contrast: dark modules on a light background; never invert. Quiet zone: leave the white border alone. And always print with a short line saying what the person gets ("Scan to leave us a review") — a naked QR code earns few scans.
Error correction matters when print quality or a logo overlay is involved: higher correction keeps the code scannable when partly obscured or worn, at the cost of a denser pattern. Medium is right for most uses; use High for small print or codes that live outdoors.