You can set the ftpd.log.filesize option to specify the maximum size of the current FTP log files (the /etc/log/ftp.cmd and /etc/log/ftp.xfer log files). By default, the maximum size of the current FTP log files is 512 KB.
The following command sets the maximum size of the current FTP log files to 1 GB:
options ftpd.log.filesize 1G