The syntax is correct, even you can restrict the file size with filesize parameter.
I tried in test, below is the output:
bash-3.2$ expdp dumpfile=exp_%U.dmp full=y compression=metadata_only logfile=PS_DW-`date '+%d%m%Y_%H%M%S'`.log parallel=4
Export: Release 18.104.22.168.0 - Production on Tue Aug 26 23:28:05 2014
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Username: / as sysdba
Dump file set for SYS.SYS_EXPORT_FULL_01 is:
Job "SYS"."SYS_EXPORT_FULL_01" successfully completed at 23:30:13
I hope it helps.
Pl post in the export / import forum - https://community.oracle.com/community/database/oracle-database-options/export_import_sql_loader_%26_external_tables
Expdp should not be invoked with SYSDBA privileges - Data Pump Export