Is your feature request related to a problem? Please describe.
It would be handy to have the size of a message calculated by the IMAP server and available without having to download the whole mail.

Most IMAP servers implement
FETCH NNN RFC822.SIZE
where NNN is the message number/uid shown in this stackoverflow article.

I think this is the way to go for getting this information.

Describe the solution you'd like
To retrieve this information, we can ask for it using the IMAP command directly or parse the result from imap_fetch_overview() depending on which protocol implementation is used.

Describe alternatives you've considered
Some dirty hack with fetching the whole message and calculating it on my own. But this doesn't scale.

Additional context
Relates to Issue #278
Pull request