QR Codes by Industry
These share one problem the other sections do not have: the code is read from a distance you do not control, and that decides the size before anything else does.
Signage and labels
Distance Decides the Size
The trade's rule of thumb is that readable range is roughly ten times the width of the symbol. It is a rule of thumb rather than a specification, and it is good enough for the thing people get wrong by an order of magnitude: a code designed at desk distance and installed to be read from across a road.
A board read from eight metres needs something around 800mm across. A gallery label read from behind a rope at a metre and a half needs roughly 150mm. Both of those are far larger than the code somebody sized on screen, which is why these pages run the arithmetic against real links rather than quoting a size.
Part of QR Code Solutions.