How to estimate the size and the hashkeys value of cluster

Question

I would love to understand more how I should calculate or estimate the size and the haskeys value when creating a cluster ?

Should it be based on table size? Column size?

I'm aware is documented Here but I still don't understand it properly

Here is my cluster

CREATE  CLUSTER clust (srvr_id INTEGER ) 
    SIZE 1024 SINGLE TABLE HASHKEYS 11;

here is my table for the cluster

CREATE TABLE Table_clust
CLUSTER clust (value2)
AS SELECT * FROM Table  ;

Best regards,

Bass