ok I found it.
// Get the page that is shown in the window.
viewPage = viewWindow.PageAsObj;
// Get the width and height of the page by reading them
// from the page sheet.
pageWidth =
viewPage.PageSheet.get_CellsSRC((short)Microsoft.Office.Interop.Visio.VisSectionIndices.visSectionObject
(short)Microsoft.Office.Interop.Visio.VisRowIndices.visRowPage,(short)Microsoft.Office.Interop.Visio.VisCellIndices.visPageWidth).ResultIU;