Postgresql 13 Change WAL segment size
Ubuntu/Debian packages for Postgres have their own layer on top of initdb and pg_ctl to control multiple instances and the integration with systemd.
The command that may be used to create an instance with specific options in Debian/Ubuntu is:
pg_createcluster [options] version name [-- initdb options]

use pg_lsclusters to see the list of already existing clusters. Possibly you want to drop the existing default cluster named main using the default segment size that you don't want, in order to have one single Postgres instance with the desired wal segment size.