how i get file size befor upload file and cancel upload

Hello all , i want check the file size befor upload and if it more than specific size i cancel upload with error message.
i can get file size befor upload by override ProcessRequest of the page

        int Filesize = context.Request.ContentLength;
        if (Filesize > 49457)
        {
            Response.Redirect("/Default.aspx");
        }
        else
        {
            base.ProcessRequest(context);
        }
but i get  page   Internet Explorer cannot display the webpage
any one know how i can fi ...