People read posts for entertainment and information.
There are plenty of mile-long posts that are entertaining (John Galt getting Newtered, the continuing CompuLife drama), or informative (FE Gold thread).
The better forum advertorials do both, and length is never a real issue, because uninterested prospects for the service will naturally stop reading when they find the material does not apply.
I guarantee that if the advertorial contained interesting and informative content, you wouldn't give much attention to the length.
People read the FE forum to see whom is fighting with who.. If the advertorial is to be successful in that forum, it would have to have CONFLICT involved. Otherwise no one would read it.