The size is defined by the variable MAX_CONTENT_LENGTH in elogd.c which is set to 10 MB. But email attachments are encoded in ASCII form, so they take useually 3-4 times the space of the binary format. If you have problems with large emails, you can use the option "Email Format = 111" which causes only the names of attachments to be included in email notifications. Users can then still click on the elog link inside the email notification and download the attachment from the elog page.