How to customize the size of WinForms TabbedGroupedMDIManager's close button?

public class CustomTabbedMDI : TabbedGroupedMDIManager
{
   public CustomTabbedMDI()
   { }
   protected override MDITabPanel CreateMDITabPanel()
   {
      return new CustomMDITabPanel(this);
   }
}
public class CustomMDITabPanel : MDITabPanel
{
   public CustomMDITabPanel(TabbedMDIManager tm) : base(tm)
   { }
   protected override Rectangle GetCloseButtonBounds()
   {
       Rectangle rect = base.GetCloseButtonBounds();
       rect.Width = 20;
       rect.Height = 20;
       return rect;
   }
}

Public Class CustomTabbedMDI Inherits TabbedGroupedMDIManager
   Public Sub New()
   End Sub
   Protected Overrides Function CreateMDITabPanel() As MDITabPanel
       Return New CustomMDITabPanel(Me)
   End Function
End Class
Public Class CustomMDITabPanel Inherits MDITabPanel
    Public Sub New(ByVal tm As TabbedMDIManager)
        MyBase.New(tm)
    End Sub
    Protected Overrides Function GetCloseButtonBounds() As Rectangle
        Dim rect As Rectangle = MyBase.GetCloseButtonBounds()
        rect.Width = 20
        rect.Height = 20
        Return rect
    End Function
End Class