thank you your reply first ...but sorry,i don't know how to use........

i copy you a part of code, think try it , but i can;t use..... it display error in Me.SizeChanged, this event can't direct to use
call me use RaiseEvent?

Private ChildForm As Form

Private Sub CandidateToolStripMenuItem1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CandidateToolStripMenuItem1.Click

        ChildForm = New personalInformation()
        ChildForm.MdiParent = Me
        ChildForm.Show()
        Me.SizeChanged += New EventHandler(AddressOf MainForm_SizeChanged)

    End Sub

 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