Knowing the Point that you want to place your element on the layer ("point" in the following code snippet) you can add the Width and Height properties of your Canvas control to its X and Y values and set the Envelope property of the ElementLayer like the following:
MapPoint mapPoint1 = Map.ScreenToMap(point); // Map is your esri map control
MapPoint mapPoint2 = Map.ScreenToMap(newPoint(point.X + canvas.Width, point.Y + (or -) canvas.Height));
ElementLayer.SetEnvelope(canvas, newEnvelope(mapPoint1, mapPoint2)); // canvas is your Canvas control