Postgresql creates directories to keep each database in. These directories aren't names, they are kept as the 'OID's of each database (oid's are "object identifiers"). This saves issues when you rename databases etc.

How then do you find out a databases size?

There is a "contrib" module called 'dbsize' which can do it for you. These modules don't get installed by default but allow extra functionality quite easily.

See the documentation for details on how to use it.