I'm not exactly sure what you're trying to do but you may try the below codes and see if it helps or not :
// Get the LayoutDocumentPane from the AvalonDock control
LayoutDocumentPane documentPane = dockManager.Layout.Descendents().OfType<LayoutDocumentPane>().FirstOrDefault();
// Get the first LayoutDocument from the LayoutDocumentPane
LayoutDocument document = documentPane.Children.FirstOrDefault() as LayoutDocument;
// Get the starting point of the LayoutDocument tab from the screen (0,0) point
Point startingPoint = document.GetFloatingWindow().PointToScreen(new Point(0, 0));
// Get the width and height of the AvalonDock LayoutDocument
double width = document.ActualWidth;
double height = document.ActualHeight;