Copy a whole database

CREATE DATABASE new_db_name WITH TEMPLATE origin_db_name;

Get list of databases

SELECT datname FROM pg_database ORDER BY datname

Get the size of all the databases

   SELECT
       t1.datname AS db_name,
       pg_database_size(t1.datname) AS db_size
   FROM pg_database t1
   ORDER BY pg_database_size(t1.datname) DESC;

The same on the command line:

PGPASSWORD=password  psql -h 10.11.12.13  -U username -c "SELECT t1.datname AS db_name, pg_size_pretty(pg_database_size(t1.datname)) AS db_size FROM pg_database t1 ORDER BY pg_database_size(t1.datname) DESC;

Get the size of a single database

SELECT pg_database_size('foobar');