our current cluster is getting bigger and bigger, currently 17 nodes,
i wonder what is the maximum and recommended cluster size with v6.1 and unicast?
no HA and only local storage used.
No problem. Our reference documentation mentions 32 physical nodes for a cluster.
You can find some scripts for migration/copying on the web, for example here (but attention, I have never used this). There is also an entry in the Wiki with a minimal roadmap.