You can specify the Page Setup dialog size and obtain the minimum form size using the PdfPageSetupDialogShowingEventArgs.FormSize and PdfPageSetupDialogShowingEventArgs.MinimumFormSize properties when the PdfViewer.PageSetupDialogShowing event is raised.
Note that the page setup dialog form will have the minimum size if the specified form size value is less than the value returned by the MinimumFormSize property.