<system.web.extensions>
       <scripting>
           <webServices>
               <jsonSerialization maxJsonLength="20971520"/>
           </webServices>
       </scripting>
</system.web.extensions>

ConfigurableJsonResult 
 public class ConfigurableJsonResult : JsonResult
    {
        public override void ExecuteResult(ControllerContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException("context");
            }
            if (JsonRequestBehavior == JsonRequestBehavior.DenyGet &&
                String.Equals(context.HttpContext.Request.HttpMethod, "GET", StringComparison.OrdinalIgnoreCase))
            {
                throw new InvalidOperationException("This request has been blocked because sensitive information could be disclosed to third party web sites when this is used in a GET request. To allow GET requests, set JsonRequestBehavior to AllowGet.");
            }

            HttpResponseBase response = context.HttpContext.Response;

            if (!String.IsNullOrEmpty(ContentType))
            {
                response.ContentType = ContentType;
            }
            else
            {
                response.ContentType = "application/json";
            }
            if (ContentEncoding != null)
            {
                response.ContentEncoding = ContentEncoding;
            }
            if (Data != null)
            {
                JavaScriptSerializer serializer = new JavaScriptSerializer();

                ScriptingJsonSerializationSection section = ConfigurationManager.GetSection("system.web.extensions/scripting/webServices/jsonSerialization") as ScriptingJsonSerializationSection;
           
                if (section != null)
                {
                    serializer.MaxJsonLength = section.MaxJsonLength;
                    serializer.RecursionLimit = section.RecursionLimit;
                }

                response.Write(serializer.Serialize(Data));
            }
        }
    }