Size against scanning distance
A rule of thumb: width at least distance divided by ten.
If someone scans from 20 cm, the rule of thumb used here asks for at least 20 mm of symbol width, quiet zone included. From 50 cm, 50 mm. This is not an ISO minimum and it does not guarantee a scan. It is a floor that filters out business-card codes meant to be read from across a room.
Module size times (modules + quiet zone on both sides) is the width. A version 4 symbol is 33 modules. With 4 modules of margin each side, that is 41 modules. At 0.8 mm per module the width is 32.8 mm.
Codes under 20 mm are flagged even when the distance field is optimistic. Small print fails more often on ordinary phone cameras.