ok la~~~ thank you , but if i want to base on parent form change size? ...
How to current location - before location ?

Private Sub MainForm_SizeChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim f As Form = DirectCast(sender, Form)
        ChildForm.Width = f.Width / 2                          <?
        ChildForm.Height = f.Height / 2                      <?
        'Or whatever
    End Sub