When files are attached to a message, due to MIME/base64-encoding, the
actual size of the email is usually something like 30% bigger than the size
the attachments.  Most people do not realize this.  Often people are aware
of attachment size limits of their mail recipients, and try to keep the
email message size within the limit, but don't take into account the
overhead of MIME-encoding.

It would be nice if the total size of an email message (headers, body,
attachments) were calculated and dynamically displayed at the bottom of the
Composer screen, perhaps right-justified in the "Show Attachment Bar" line.
 This would make the actual message size obvious.