I often attach a file to an email and then realize
that it is too big for whatever mailing list I am
posting too.

It would be really useful if I could tell
evolution (via the context menu) to compress the
attachment with (gzip|bzip2|zip) before sending.
That way I wouldn't have to dig around finding the
file again, compress it (remembering to -k to keep
the original), attach it and then remember to
delete the compressed version once I'd sent it.

Perhaps MIME would even allow you to compress
attachments transparently to me and the
recipients? Not sure that is a great idea though.