Finally after exploring all the metrics/statistics info from the Solr admin overview page, below information helped to find the total size:
1) document of solr are composed of indexes & files compressed
2) the size of your solr core per node is to get in solr admin page
3) From the admin page corresponding core overview section provides Num of Docs & Total Size (including index & stored files)
4) Similarly get the size info from all Solr Nodes