To get an overview about how much space is taken by what database, use:

SELECT
    pg_database.datname,
    pg_size_pretty(pg_database_size(pg_database.datname)) AS size_pretty,
    pg_database_size(pg_database.datname) AS size
    FROM pg_database
    ORDER BY pg_database_size(pg_database.datname) DESC;

To get more details, use:

-- get dead tuples, last vacuum, last analyze
SELECT pg_size_pretty(pg_relation_size(relid)) AS tablesize, schemaname, relname, n_live_tup, n_dead_tup, last_vacuum, last_autovacuum, last_analyze, last_autoanalyze
FROM pg_stat_user_tables
--WHERE relname = 'fct_encounter'
--WHERE schemaname = 'dm'
ORDER BY n_live_tup DESC;

SELECT
   schemaname,
   relname as "Table",
   pg_size_pretty(pg_total_relation_size(relid)) As "Size",
   pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as "External Size"
   FROM pg_catalog.pg_statio_user_tables 
   ORDER BY pg_total_relation_size(relid) DESC;

It will show 3 columns:

    Table – The name of the table
    Size – The total size that this table takes
    External Size – The size that related objects of this table like indices take

If you want to dig in deeper and see the size of all objects (at least of 64kB):

SELECT
   relname AS objectname,
   relkind AS objecttype,
   reltuples AS "#entries", 
   pg_size_pretty(relpages::bigint*8*1024) AS size
   FROM pg_class
   WHERE relpages >= 8
   ORDER BY relpages DESC;

This will show 4 columns:

    objectname – The name of the object
    objecttype – r for the table, i for an index, t for toast data, ...
    #entries – The number of entries in the object (e.g. rows)
    size – The size of the object