How I can monitor Amazon RDS Postgresql table size once the `hot_standby_feedback` is on
force manually an autovacumm policy on mergency situations


This is not going to work. hot_standby_feedback doesn't prevent autovacuum from running. It lets it run but prevents it from removing rows that are not "dead enough".  No amount of vacuuming is going to help if you don't get the long running queries on the replicas to go away first.

You can use pg_total_relation_size to get the table size.  You can take a query like the below, and just plug it into our favorite monitoring solution.

select pg_total_relation_size('pgbench_accounts');


But that still leaves the question of what to do about it.