The documentation needs to cover this:

If --cluster-size=N is specified then the cluster will not become active until it has N members. This is useful if your cluster members have persistent stores, since it allows all the members to compare the status and select the correct store image to start up with.

If a client connects to the cluster while there are < N members, the client will hang opening the connection until all N members have joined the cluster, then it will proceed as normal. 

If connection heartbeats are enabled they can interrupt this hang XREF to heartbeats