QPrintPreviewDialog constructs a QPageSetupDialog for the "Page Setup" action; the latter's constructor ultimately calls QPageSetupWidget::setPrinter with a nullptr for the printDevice parameter, which means it always uses A4 paper size instead of whatever default paper size set in lpoptions.

Attached is a patch that should fix the issue; basically it complements the change in commit ff67dedaaff2dc68571ca9c8c6c6eeaaf084eae4 (which made the Page tab on QPrintPropertiesDialog use the printDevice default paper size).