Public Partial Class MainForm Inherits Form Private ChildForm As Form Public Sub New() InitializeComponent() ChildForm = New ChildForm() ChildForm.MdiParent = Me ChildForm.Show() Me.SizeChanged += New EventHandler(AddressOf MainForm_SizeChanged) End Sub Private Sub MainForm_SizeChanged(sender As Object, e As EventArgs) Dim f As Form = DirectCast(sender, Form) ChildForm.Width = f.Width / 2 ChildForm.Height = f.Height / 2 'Or whatever End Sub End Class