Here's hoping I understand your question correctly, this is what I found in the man pages for journald config: journald.conf(5)
CONFIGURATION DIRECTORIES AND PRECEDENCE
The default configuration is defined during compilation, so a configuration file is only needed when
it is necessary to deviate from those defaults.
By default, the configuration file in
contains commented out entries showing the defaults as a guide to the administrator. This file can
be edited to create local overrides.
This would mean that you have all the defaults already in the config file mentioned. However, some options, specifically the ones you're looking for about max size, are not filled in.
There's a difference between "persistent" and "auto/runtime" storage mode.
Here's what I find below:
Enforce size limits on the journal files stored. The options prefixed with "System" apply to the
journal files when stored on a persistent file system, more specifically
options prefixed with "Runtime" apply to the journal files when stored on a volatile in-memory
file system, more specifically
/run/log/journal. The former is used only when
/var is mounted,
writable, and the directory
/var/log/journal exists. Otherwise, only the latter applies. Note
that this means that during early boot and if the administrator disabled persistent logging,
only the latter options apply, while the former apply if persistent logging is enabled and the
system is fully booted up. …
and followed by this:
The first pair defaults to 10% and the second to 15% of the size of the respective file system,
but each value is capped to 4G. If the file system is nearly full and either
RuntimeKeepFree= are violated when systemd-journald is started, the limit will be raised to the
percentage that is actually free. …
The man-page for journald.conf goes on about these settings so I think it'd be best and more up-to-date for the version you run if you check the man page that's actually on your own system.
About log rotation, I'm assuming you're asking about the journal logs and not about the logrotate-service? The former is also in the man-pages and it's called "vacuum cleaning" while the latter is a different service.
Systemd offers the possibility to replace the old logrotations via cron jobs with "timer" services to do the same.
The timers can be found like this:
sudo systemctl list-timers --all --no-pager and timers can be enabled using normal "sytemctl enable" commands, etc.