Start with the use distance
A tabletop card scanned from arm’s length can use a smaller code than a poster scanned across a room. There is no universal minimum because camera quality, lighting, payload length, print process, and surface all matter.
Protect the quiet zone
A QR symbol needs a clear margin around every side. DENSO WAVE specifies a quiet zone four modules wide. Qwero includes that margin automatically; do not crop it away or place borders and artwork inside it.
Use a conservative proof
For small counter cards, a code around 1.25–1.5 inches wide is a useful starting point. Increase it for dense URLs, difficult lighting, textured materials, or greater viewing distance. Then test the physical proof with several phones.
Sources
Primary references used for factual guidance: