Resize MDI subwindows

[8Observer8]

Hi,

I want to resize subwindows of simple MDI application:

#include "MainWindow.h"
#include "ui_MainWindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    m_firstWindow = new FirstWindow;
    m_secondWindow = new SecondWindow;
    m_thirdWindow = new ThirdWindow;
 
    m_firstWindow->resize( 500, 500 );
 
    ui->mdiArea->addSubWindow( m_firstWindow );
    ui->mdiArea->addSubWindow( m_secondWindow );
    ui->mdiArea->addSubWindow( m_thirdWindow );
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

#include "MainWindow.h"
#include "ui_MainWindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    m_firstWindow = new FirstWindow;
    m_secondWindow = new SecondWindow;
    m_thirdWindow = new ThirdWindow;

    m_firstWindow->resize( 500, 500 );

    ui->mdiArea->addSubWindow( m_firstWindow );
    ui->mdiArea->addSubWindow( m_secondWindow );
    ui->mdiArea->addSubWindow( m_thirdWindow );
}

MainWindow::~MainWindow()
{
    delete ui;
}

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But it doesn't work:
280.png

It is the project: https://github.com/8Observer8/SimpleMDIApplication

Thank you!