you do need to specify the --cluster-ramsize option on cluster init. However there is a bug that return the error even when you specify the cluster-ramsize. I logged an issue for tracking this:
https://issues.couchbase.com/browse/MB-18092
./couchbase-cli cluster-init -c localhost:8091 -u couchbase123 -p couchbase123 --cluster-init-port=9090 --cluster-ramsize=1000
this should return the following. and pls ignore the error because the operation succeeded.
ERROR: option cluster-ramsize is not specified
SUCCESS: init/edit localhost