<system.web.extensions>
    <scripting>
      <webServices>
        <jsonSerialization maxJsonLength="1024000" />
      </webServices>
    </scripting>
</system.web.extensions>

return new ContentResult
                {
                    Content = new System.Web.Script.Serialization.JavaScriptSerializer { MaxJsonLength = Int32.MaxValue }.Serialize(data),
                    ContentType = "application/json"
                };