showModalDialog popup window size problem

anyone help me to open poup window using window.showModalDialog. 
This is my code already i use. But i cant give a size for popup window.
My default browser IE 6 and 7.

 
 String popupScript = "<script language='javascript'>" + "window.showModalDialog('MODisplay.aspx', '', 'width=400; height=200; resizable=yes; scrollbars=yes; top=300; left=300')" + "</script>";
if (ScriptManager.GetCurrent(base.Page).IsInAsyncPostBack)
                        {
                            ScriptManager.RegisterStartupScript(this, this.GetType(), "PopupScript", popupScript, false);
     ...