Apologies for the necro, but to clarify, the modification to /etc/systemd/journald.conf to limit the maximum size of the journald logs is as follows.
As root, modify the file to uncomment
#SystemMaxUse=
and set a max threshhold in MB (M), GB (G), etc.
SystemMaxUse=512M
Save the file and restart the journald service.
$ sudo systemctl restart systemd-journald.service