First things first. This comes up frequently here... "doubt" has the connotation of skepticism, or mild disbelief. "question" means query, request for clarification, etc. You don't need a doubt solved, you need a question resolved.

Now on to the questions ... at least the ones I can answer...

1. From what I can tell, MIME::Lite imposes no artificial limitations on the size of individual attachments. However, given the fact that most mailboxes have their own system administrator imposed capacity limits, common sense will dictate that attachments be kept to a reasonable size. What constitutes "reasonable size" is somewhat of a moving target. Over time, mailbox size policies seem to grow more generous, and the proliferation of broadband connectivity makes it easier for people to download large messages. Nevertheless, it makes sense to keep attachment sizes as small as practical.

   If you find that you need to send truely massive files, consider another approach; FTP for example.

2. When sendmail is used as the mail transport, and when it is configured without the -oi option, a single '.' on a line can act as the message terminator. I believe this can expose a security problem. That is why the module's POD is alerting you to the danger of a poorly configured MTA, and recommends the -oi option be used with sendmail. The module itself is as safe as the configuration of the system on which it's being used.