Make absolute layout as same size as JFrame at design time for convinience

I am developing a Java Swing application. I have a small issue on how to resize the layout in a JFrame to the same size of the JFrame in the Design tab in Eclipse.

The layout's space is not enough to effectively design my intended user interface. I can add items in a crammed manner and those items display in the final output but at design time I would like the layout to expand as much as the JFrame. Following image would describe my problem:

I tried this code but it does not seem to work:

getContentPane().setPreferredSize(new Dimension(800, 600));

This is my code:

public class ExployeeScreen extends JFrame {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    ExployeeScreen frame = new ExployeeScreen();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public ExployeeScreen() {
        setBounds(100, 100, 450, 300);
        setExtendedState(JFrame.MAXIMIZED_BOTH);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().setLayout(null);
        getContentPane().setPreferredSize(new Dimension(800, 600));

    }

}

Any help regarding this is highly appreciated. Thanks :)