Based on https://dba.stackexchange.com/a/134574/75929

psql_show_biggest_partitions.sql

SELECT nspname || '.' || relname AS "relation",
       pg_total_relation_size(C.oid) AS "total_size"
FROM pg_class C
LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
LEFT OUTER JOIN pg_inherits I ON (C.relfilenode = I.inhrelid OR C.relfilenode = I.inhparent)
WHERE nspname NOT IN ('pg_catalog', 'information_schema')
    AND C.relkind <> 'i'
    AND nspname !~ '^pg_toast'
    AND I.inhrelid is null
UNION ALL
select nspname || '.' || relname AS "relation",
       sum(pg_relation_size(inhrelid)) AS "total_size"
FROM pg_inherits  I
LEFT JOIN pg_class C ON (C.oid = I.inhparent)
LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
GROUP BY  nspname,relname
ORDER BY 2 DESC LIMIT 10
;