How to estimate the size of a PostgreSQL DB to be dumped through pg_dump Similar to this question about MySQL -- 
I need to find out the size of a dump file prior dumping but for a PostgreSQL Database. I could not find any reference to this in the documentation
A very rough estimate is to compute the size of all tables with:

select pg_size_pretty(sum(pg_table_size(tablename::regclass))) 
from pg_tables 
where schemaname not in ('pg_catalog','information_schema');