Here's a way to get a print preview dialog (or perhaps any resizeable dialog) maximised.
PrintPreviewDialog dlg = new PrintPreviewDialog();
dlg.Document = doc;
dlg.Show();
//Make full screen
//cant use WindowState as its not a form
dlg.DesktopBounds = Screen.FromControl(dlg).WorkingArea;
dlg.BringToFront();
Although some times it disappears behind whatever form had focus last.