Can't get minimum size of QWindow including frame

Looking at the QWindow documentation page here:

https://doc.qt.io/qt-6/qwindow.html

I'm not able to find how to get the minimum size of a window including the frame/title bar.

In a very simple C++ example like this:

#include <QWindow>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWindow w;
    w.show();
    return a.exec();
}

I want to be able to get an attribute on the QWindow that tells me the minimum size including the title bar.

I noticed I'm only able to scale the window to a certain size (approx. 120px x 30px), but I'm not sure why exactly.