PostgreSQL : Get total pages of an object

SELECT pg_relation_size(<object_id>, 'main')/8192);

The pg_relation_size gives the disk space occupied by the object. Since each page is of 8kB, the pg_relation_size size is divided by 8192 to give the page count.

PostgreSQL : Data Object sizes

 Size of User Tables

=# SELECT
   relname as "Table",
   pg_size_pretty(pg_total_relation_size(relid)) as "Total Size",
   pg_size_pretty(pg_relation_size(relid)) as "Relation 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
;
      Table       | Total Size | Relation Size | External Size
------------------+------------+---------------+---------------
 load_test        | 1883 MB    | 1776 MB       | 107 MB
 pgbench_accounts | 1121 MB    | 961 MB        | 161 MB
 pgbench_history  | 8656 kB    | 8656 kB       | 0 bytes
 pgbench_tellers  | 80 kB      | 40 kB         | 40 kB
 pgbench_branches | 24 kB      | 8192 bytes    | 16 kB
(5 rows)

 Size of User Indexes

=# SELECT
   indexrelname as "Index",
   pg_size_pretty(pg_total_relation_size(indexrelid)) as "Size",
   pg_size_pretty(pg_total_relation_size(indexrelid) - pg_relation_size(indexrelid)) as "External Size"
   FROM pg_catalog.pg_statio_user_indexes ORDER BY pg_total_relation_size(indexrelid) DESC;
         Index         |  Size  | External Size
-----------------------+--------+---------------
 pgbench_accounts_pkey | 161 MB | 0 bytes
 load_testidx          | 107 MB | 0 bytes
 pgbench_tellers_pkey  | 40 kB  | 0 bytes
 pgbench_branches_pkey | 16 kB  | 0 bytes
(4 rows)

 Functions

pg_relation_size(oid) - returns disk space occupied by the table or index of the given oid.

pg_total_relation_size(oid) - returns total disk space of the table of given oid, including indexes and toasted data

Find the list of directories with their Sizes

Get Size of all sub-directories

Command: 

du -sh <directory_path>/*

Example:
$ du -sh /home/Beena/*

4.0K /home/Beena/Desktop
4.0K /home/Beena/Documents
64M /home/Beena/Downloads
4.0K /home/Beena/Music
4.0K /home/Beena/Pictures
4.0K /home/Beena/Public
4.0K /home/Beena/Templates
4.0K /home/Beena/Videos

Get all files and sub-directories More than a specified size

Command: 

find <dir_path> -size +<size> -ls

Example:
$ find /home/Beena/ -size +10M -ls

Get all files and sub-directories Less than a specified size

Command: 

find <dir_path> -size -<size> -ls

Example:
$ find /home/Beena/ -size -10M -ls