I know some may disagree, and that's ok, but, in MY opinion,
emails are not meant to be salespages. They are meant to be
intros to whatever it is I am trying to sell (be it a product, a
service, or just simple information).
Therefore, I always try to treat emails like small introductory
"snippets" and then providing a link to a page on my website
which goes into more detail (or an affiliate link to the item(s)
HOWEVER, the email needs to be extremely captivating, since
you are trying to grab and KEEP their attention with the goal
of getting them to click on the link to your site.
An exception to this, however, is when an email is all that is
needed. Many times, my ONLY goal is to provide a bit of info,
in which case, the email should be as long as it needs to be.
If the reader is truly interested in the information you are
providing, s/he will read as much of it as s/he needs to.