Eudora gives sizes in inboxes and outboxes. Two things can add invisibly to length: 1) headers, which add about 1k, and 2) HTML coding, which can add 5k or more to a post.
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Just so.
I heartily recommend Eudora (Outlook Express users take note - see URL below for Eudora info).
I think most super-sized posts are the result of folks copying and pasting web originated text directly into their mailer of choice (often a web-based widget such as Yahoo!'s) and simply pressing "send" without awareness of the post's heft.
Also, as you say, headers and HTML coding adds overhead so it's a good idea for everyone to set their default outgoing format to *plain text* when posting to avoid this.
For those situations when members want to post text from a long piece as a teaser to a URL, a bit of massaging using a frills-free editing app (if you're not a Eudora or Mozilla mail user) will give you the ability to edit for coherency while getting a stripped-down file size count before pasting into your mailer for submission.
Eudora mail application -
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DRM