How can I query the size of my Postgres database?

Resolution

There are several handy Postgres functions that you can use to query and format the database size:

pg_size_pretty: Converts a size in bytes expressed as a numeric value into a human-readable format with size units.
pg_database_size: Disk space used by the database with the specified OID.
current_database: Name of current database

Put these together and you can write a query like so:

sushi_db::DATABASE=> SELECT pg_size_pretty(pg_database_size(current_database())) as size;
 size  
-------
 13 MB
(1 row)