The 10:1 rule says the scan distance should be at most ten times the QR's side length. A 10 cm code reliably scans from about 1 meter. A 1-meter code on a billboard works from 10 meters. It's a floor, not a target — bigger always scans better.
Why ten to one?
It's roughly the point at which a phone camera can still resolve individual modules (the little squares) given typical sensor resolution, focus range, and the camera shake of someone holding a phone. Under 10:1, scans are basically instant. Past it, they get flaky.
When to print bigger than the minimum
Anytime the print isn't pristine. Faded toner, glossy laminate, curved surfaces, and dim lighting all eat into the effective scan distance. Older phones do too. And if you're encoding a lot of data — a long URL, a full vCard — the QR has more modules to fit in the same physical size, so each module is smaller and the code becomes pickier. (The Capacity Calculator shows you exactly how many modules your data needs.)
Print resolution and pixel size
For paper, 300 DPI is the standard. That's about 11.8 pixels per millimeter of QR. For posters and billboards, 150 DPI is plenty because nobody's reading a billboard from 30 cm away. Below 150 DPI on a small code, the module edges start to blur into each other and scanners get confused.