Unsolved Maximizing the tool to full screen

[serendipity2]

I found some code for maximizing my GUI:
void MainWindow::mouseDoubleClickEvent(QMouseEvent *e) {
QWidget::mouseDoubleClickEvent(e);
if(isFullScreen()) {
this->setWindowState(Qt::WindowMaximized);
} else {
this->setWindowState(Qt::WindowFullScreen);
}
}

It works for the main window fine. How do I arrange my buttons that they fit in the full screen optic?
Where can I look it up?
What is the best solution to do this?

[jsulm]

@serendipity2 Use layouts: http://doc.qt.io/qt-5/layout.html
http://doc.qt.io/qt-5/examples-layouts.html