The background box of the ChartText object is sized to the text. There is no way to set the background rectangle independent of the associated text.
You could probably use the RTTextFrame object to acheive the same result. It places an opaque frame behind a ChartText object that can be sized independent of the text object.
ChartAttribute attrib3 = new ChartAttribute (Color.Black, 1,DashStyle.Solid, Color.LightBlue);
ChartText charttext = new ChartText(pTransform1, titlefont,"RT Chart Text", 0.5, 0.1, ChartObj.NORM_GRAPH_POS);
charttext.ChartObjAttributes = attrib3;
RTTextFrame rttext = new RTTextFrame(charttext);
rttext.MinTextBoxWidth = 400;
chartVu.AddChartObject(rttext);