will there be any impact of ingested contents when we do resize of cluster size of Retrieve and Rank service from 2 units to 4 units? Also how can we get current index size of RnR cluster?
You can get the index size by submitting a GET to /v1/solr_clusters/{solr_cluster_id}/stats
There's more info about this API at https://www.ibm.com/watson/developercloud/retrieve-and-rank/api/v1/#get_statistics (including the way to invoke it if you're using one of the SDKs)
As for impact of resizing - no, as far as I know that should be fine.