Qt: Set size of QMainWindow

Question

I'm new to Qt, so I wonder whether there is a way to set the size of a QMainWindow to (for example) 70% of the user's desktop.
I tried the stretch factor but it didn't work. QWidget::setFixedSize worked but only with a pixel number, I think.

Answer 1

Thanks to Amir eas. The problem is solved. Here's the code for it:

#include <QDesktopWidget>
#include <QMainWindow>
...
QDesktopWidget dw;
MainWindow w;
...
int x=dw.width()*0.7;
int y=dw.height()*0.7;
w.setFixedSize(x,y);