Set the Dock() property of the TabControl to Fill, then handle the SelectedIndexChanged() event:
Private Sub TabControl1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles TabControl1.SelectedIndexChanged
Select Case TabControl1.SelectedIndex
Case 0
Me.Size = New Size(300, 200)
Case 1
Me.Size = New Size(400, 400)
End Select
End Sub