Thank you all,
Actually, "the documents" i mentioned in my question, was a talk in youtube seen long time ago and could not find it.
Also noticing that a lot of companies like Netflix built hundreds of Clusters each having 10s of nodes and saying that its much stable, i just concluded that big cluster is not recommended. I see some of the reasons in your answers:
the problem with dynamic snitch and probability of node failures that simply increases with more nodes in cluster which even could cause cluster outage.