Get Database size

SELECT pg_size_pretty(pg_database_size('db_name_here')) As fulldbsize;

Get size of all tables in a schema

select pg_size_pretty(CAST((SELECT
SUM(pg_total_relation_size(table_schema || '.' || table_name) )
FROM information_schema.tables
WHERE table_schema = 'schema_name_here') As bigint) )  As tables_schema_size;

Get sizes of a table

Full_size = Size of table, its toasted tables and indexes
Table_only = Size of the table only

SELECT pg_size_pretty(pg_total_relation_size('table_name_here')) as Full_size, pg_size_pretty(pg_relation_size('table_name_here')) as Table_only;