Philipp Haselwarter put forth on 1/15/2011 8:32 PM:
,---- | More than 97% of all e-mails sent over the net are unwanted, according | to a Microsoft security report.[39] | | MAAWG estimates that 85% of incoming mail is "abusive email", as of the | second half of 2007. The sample size for the MAAWG's study was over 100 | million mailboxes.[40][41][42] | | Spamhaus estimates that 90% of incoming e-mail traffic is spam in North | America, Europe or Australasia.[43] By June 2008 96.5% of e-mail | received by businesses was spam.[18][unreliable source?] `----
I just have a tiny set of 4k spam mails, but they have an avg size of 39KB, ie well above 4KB.
This discussion has been in the context of _storing_ user email. The assumption is that an OP is smart/talented enough to get his spam filters/appliances killing 99% before it reaches intermediate storage or mailboxes. Thus, in the context of this discussion, the average size of a spam message is irrelevant, because we're talking about what goes into the mail store.
If you're storing significantly more than 1% of spam you need to get that under control before doing any kind of meaningful analysis of mail storage needs.
-- Stan