vyos@vyos# set system syslog file myfile archive size 
Possible completions:
   <text>               Size of log files in kbytes (default: 256)

size={{ file_options.archive.size | int // 1024 }}k

vyos@vyos# cat /etc/logrotate.d/vyos-rsyslog-user
### Autogenerated by system_syslog.py ###
/var/log/user/myfile {
  missingok
  notifempty
  create
  rotate 5
  size=0k
}