Understanding "max_wal_size" and "min_wal_size" parameters default values from postgresql.conf file
These are 16 MB WAL segments by default. The manual:


  The system physically divides this sequence into WAL segment files,
  which are normally 16MB apiece (although the segment size can be
  altered when building PostgreSQL)


So, it's just default values as advertised:

select name, setting, setting::int * 16 || 'MB' AS setting_in_mb
from pg_settings 
where name in ('min_wal_size', 'max_wal_size');


unit in pg_settings is present when it's an actual base unit like second or MB. In this case the "unit" would be "WAL segments", which can be configured before compiling Postgres. The unit column is just NULL for those.