How Big Can a QR Code Be?
There's no hard upper limit on the physical print size of a QR code — SVG or vector output scales infinitely, and QR codes have been printed on billboards and building sides. The real constraint is scanning distance: a bigger physical code needs to be scanned from further away, and camera autofocus has practical limits at extreme range.
This is really two separate questions people mean when they ask "how big can a QR code be": physical print size (effectively unlimited with vector artwork) and data capacity (capped by the spec — see the Encoding Capacity entry). For a billboard-sized code, keep the encoded content short — a shortened URL, not a paragraph — since more data means a denser pattern that needs even more distance-appropriate size to stay scannable.
For everyday printing, the practical rule is closer to "how small," not "how big": SVG export scales cleanly to any size without quality loss, so oversizing is rarely the problem — undersizing for the intended scan distance is.