Document doc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
Point2d screenSize = (Point2d)Autodesk.AutoCAD.ApplicationServices.Application.GetSystemVariable("SCREENSIZE");
System.Drawing.Point upperLeft = new System.Drawing.Point(0, 0);
System.Drawing.Point lowerRight = new System.Drawing.Point((int)screenSize.X,(int)screenSize.Y);
Point3d upperLeftWorld = ed.PointToWorld(upperLeft, 0);
Point3d lowerRightWorld = ed.PointToWorld(lowerRight, 0);
The variable Scrrensize gives you the size of the current view in Pixel.
The PointToWorld Function gives you the "bounding box" of the screen in World Coordinates.
With this data you should be able to calculate everything you want 🙂