[jiggy2011]

The problem is that you are trying to enforce it on top of an existing platform with a use case for which it doesn't really make sense.

If you really don't want to receive long emails from strangers you can always add a filter in your mail setup that deletes any email over 500 chars long from people not in your address book with an autoreply that says "I don't read anything over 500 chars long".