Thanks Mike!I got it done!
In window size changed event,I made it like this:
Private Sub MainWindow_SizeChanged(ByVal sender As Object, ByVal e As System.Windows.SizeChangedEventArgs) Handles Me.SizeChanged
If WindowState = WindowState.Maximized Then
txt.Width = Windows.WindowState.Maximized
txt.Height = Me.ActualHeight : txt.Width = Me.ActualWidth
End If
End Sub
It's working fine!