If you want them to apply immediately, you may want to force log rotation using:
logrotate -f your_logrotate_config_files
Otherwise, logrotate normally runs as a daily cron job.
This means it is in fact restarted each day, and reads the config files when it is started.
So, your config changes will take effect the next day at the latest.
/etc/logrotate.conf (and/or your own config files)