Create this file (and possibly its parent directory):
/etc/systemd/journald.conf.d/max-size.conf
With contents:
[Journal]
SystemMaxUse=100M
 The space will be freed after you reboot or run sudo systemctl restart systemd-journald
 This will keep your journald log small now and forever. Because the config is stored in a dedicated file, there is no editing mess and it's easy to remove the config later (or see what exactly have you changed in the past). You can also specify maximum journal entries time with: MaxRetentionSec=3day
 
                