Have you tried this?
void QWidget::showFullScreen () [slot]
Shows the widget in full-screen mode.
Calling this function only affects windows.
To return from full-screen mode, call showNormal().
Full-screen mode works fine under Windows, but has certain problems under X. [...]
X11 window managers that follow modern post-ICCCM specifications support full-screen mode properly.
See also showNormal(), showMaximized(), show(), hide(), and isVisible().