What is the average size of an email message?  I'm looking for a
research source, not a marketing source.  An example of a good source
for the answer is:

http://www.sims.berkeley.edu/research/projects/how-much-info/internet.html

but it's outdated...can you find something more current circa 2003, 2004?

The Berkeley study you mentioned was updated in 2003. This may be of interest:

"About 31 billion emails are sent daily, on the Internet and
elsewhere, a figure which is expected to double by 2006 (source:
International Data Corporation (IDC). The average email is about 59
kilobytes in size, thus the annual flow of emails worldwide is 667,585
terabytes."

School of Information Management, Berkeley: How Much Information? 2003
http://www.sims.berkeley.edu/research/projects/how-much-info-2003/execsum.htm#summary

Thanks very much, pinkfreud.  You're always helpful.