Launch the QPageSetupDialog and request the paper size selected by user:

QPageSetupDialog myPageSetupDialog(myQPrinter, this);
if (myPageSetupDialog.exec() == QDialog::Accepted) {
QPrinter::PaperSize size = myQPrinter->paperSize();

QPrinter::paperSize() returns the paper size based on the lookup table in %QTDIR%\src\gui\painting\qprintengine_win.cpp, which rather than mapping some unusual paper sizes to QPrinter::Custom, actually maps them to other paper sizes (function mapDevmodePaperSize(int s) on qprintengine_win.cpp).

This works ok if the original paper size either maps identically or doesn't appear in the table. But if the user selects a paper size that is non-identically mapped, QPrinter::paperSize() returns incorrect paper size. E.g. if 'A4 Extra' selected on QPageSetupDialog --> QPrinter::paperSize() returns 'A4'.

These alternative sizes get returned also if you request the actual dimensions as it maps to the QPrinter::PaperSize alternative before looking up the dimensions.

Note: if QPrintPreviewWidget is used with same QPrinter instance, it returns the paper size correctly (from the DEVMODE structure held inside the QPrinter code hierarchy).