Printing resolution indipendent, in millimeters

Hi to all,
I want to draw and print some basic shapes like squares and I need the way to use millimeter unit in natural way.
For example I need to print an exact 10 * 10 mm square
I expect to print a square starting a t x=20 mm, y =20 mm with 10 mm side.. .but something wrong about dimension of printer output.
Could someone please put me in the rigth way or provide a little example about conversion to the measure unit used in Qpainter and the unit used in Printer?

Many thanks in advance.
this is the code I'm using:
Code:

QPrinter printer(QPrinter::HighResolution); printer.setResolution(600); printer.setPaperSize(QPrinter::A4); printer.setOrientation (QPrinter::Portrait); printer.setNumCopies(1); QPainter painter; painter.begin(&printer); painter.drawRect(20,20,10,10); painter.end();