How do I delete, on a daily basis, the mail statistics logs ?
Mail statistics are stored in a MySQL database located at /var/lib/mysql/eximstats. You'll need to remove the entries from this database to clean it up.
Alternatively, mail logs located in /var/log, exim_mainlog, exim_rejectlog, and exim_paniclog will need to be wiped clean (or /exim/mainlog, /exim/rejectlog, /exim/paniclog for FreeBSD®) daily.
A cronjob can easily execute a script that wipes either of these daily.