Why specify a length for character varying types
My understanding is that having constraints is useful for data integrity, therefore I use column sizes to both validate the data items at the lower layer, and to better describe the data model.
Some links on the matter:
My understanding is that this is a legacy of older databases with storage that wasn't as flexible as that of Postgres. Some would use fixed-length structures to make it easy to find particular records and, since SQL is a somewhat standardized language, that legacy is still seen even when it doesn't provide any practical benefit.
Thus, your "make it big" approach should be an entirely reasonable one with Postgres, but it may not transfer well to other less flexible RDBMS systems.
The documentation explains this:
If character varying is used without length specifier, the type accepts strings of any size. The latter is a PostgreSQL extension.
The SQL standard requires a length specification for all its types. This is probably mainly for legacy reasons. Among PostgreSQL users, the preference tends to be to omit the length specification, but if you want to write portable code, you have to include it (and pick an arbitrary size, in many cases).