QPrinter printer(QPrinter::HighResolution);
printer.setPageSize(QPrinter::A4);
QPrintDialog printDialog(&printer, this);

QPainter painter(&printer);

QRect rect = painter.viewport();
QSize size(590,840); 
size.scale(rect.size(), Qt::KeepAspectRatio); 
painter.setViewport(rect.x(), rect.y(), 
	                        size.width(), size.height()); 
			painter.setWindow(0,0,590,840); 

//<<<<<<
qDebug() << "Viewport:" << painter.viewport();
// <<<<<<

painter.setRenderHint(QPainter::SmoothPixmapTransform, true);
painter.begin();			
QImage img; 
img.load("test.jpg");
img = img.scaled(520,450, Qt::KeepAspectRatio, Qt::SmoothTransformation);
painter.drawImage(0, 0, img);
painter.end();