The short answer is a ratio, not a number: the code should be about one tenth of the distance it will be scanned from.
Scanned from 30 cm (a table tent, a business card): 3 cm minimum
Scanned from 1 m (a shop window, a counter sign): 10 cm
Scanned from 3 m (a wall poster): 30 cm
Scanned from 10 m (a banner, a vehicle): 1 m
Treat those as floors, not targets. A code that is comfortably too big costs nothing; one that is 15% too small fails silently and you never hear about it.
The three things that break a technically correct size
The quiet zone. A QR code needs clear space around it — roughly four modules' worth, where a module is one of the little squares. Designers trim it constantly to fit a layout. A code with artwork crowding its edge fails at any size.
Module size after printing. What matters physically is how big one module ends up. Below roughly 0.5 mm per module, ink spread on absorbent stock starts merging neighbouring modules. If your code encodes a lot of data it has more, smaller modules at the same overall size — which is one practical reason to point a code at a short URL rather than a long one.
The surface. Curved bottles, textured card, anything laminated glossy under a shop light. Curvature distorts the grid; gloss throws back the light the camera needs. On a curved surface, go a size up and keep the code away from the steepest part of the curve.
Test it the way a customer will
Print one, at final size, on the final material. Then scan it: at the real distance, at an angle, in the actual lighting, on the oldest phone in the office. A code that works on your desk under a lamp tells you nothing about a rainy Tuesday outside a café.
If you are printing a run of thousands, the ten minutes this takes is the cheapest insurance available.
