The sendmail-largeboxes.cof file is used to display warnings for large mailboxes. The default configuration is hard coded to consider 40MB+ as large. Users will receive email warnings that they have exceeded the 40MB mailbox size. You can change the default 40MB to a different value. On my Fedora Core 4 installation the file is located in /usr/share/logwatch/default.conf/services/sendmail-largebooxes.conf.   The variable to change is $sendmail_largeboxes_size = 40MB.