In my system the log space was getting big (more than 4Gb). Since nowadays we have these small solid state disks this can be a problem.

sudo journalctl --vacuum-size=500M

Moreover, you can activate log rotation with:

sudo systemctl kill --kill-who=main --signal=SIGUSR2 systemd-journald.service

and you'll need to restart the service:

sudo systemctl restart systemd-journald.service

Finally, if you want to check the size of your journaling logging space:

journalctl --disk-usage

Or, in the old-fashioned (and more 'physical') way:

du -h /var/log/journal/

No more complaints about the size of the root filesystem, for now.