pg_database_size(oid)

I have just started with postgres and after going through manual nearly for 2 hours, I need help.

I have created a database, which I plan to load with several tables. I am trying to find the size of the databases and came across pg_database_size(oid) function. Since it requires databse oid, I thought there must be a view where you get it - then came across "pg_database" table and still all in vain. The "pg_database" table lists all databases in the server but won't provide me the oid for pg_database_size(). 

I noticed this being true for almost all reference views. For example, pg_tablespace lists the tablespaces but no tablespace oid.

Function pg_tablespace_databases (tablespace_oid) , needs tablespace_oid, but where I could get it from?

Please help!

-Thanks,

DP.