A business-card QR is scanned from inches away.
A sign may be scanned from several feet away.
Physical size needs to match the real viewing distance.
Gloss, curvature, texture, folds, seams, and low-quality printing can all affect scan reliability.
Small formats require more restraint.
Do not squeeze a QR into the last open corner of a design.
The surrounding message should explain what the person gets by scanning.
A one-night event flyer may need a QR for days.
A sign or product package may remain in circulation for years.
The longer the lifespan, the more valuable destination flexibility becomes.