How big should a QR code be?
Print a QR code at least one-tenth as wide as the farthest distance it will be scanned from: a code read from 25 cm away needs about 2.5 cm, a poster read from 3 metres needs about 30 cm. Below roughly 2 x 2 cm most phone cameras struggle regardless of distance.
The 10:1 rule of thumb covers most real placements: business cards and menus are scanned close, posters and windows from further back, so size the code for the farthest realistic reader. Denser codes (long URLs, high error correction) pack more modules into the same square, which effectively raises the minimum size; short links keep codes sparse and scannable.
Size isn't the only physical factor: keep the quiet zone clear and print sharp. The size calculator does the distance arithmetic for you, and the scan distance reference has the full table.
See also
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