Having published a Newsletter for over 6 years we've learned a few "tricks". Probably most important one is that people want to get their information as quickly as possible. They are not willing to wade through tomes of information, no matter how well written, to get ideas presented in article.
If two identical newsletters are published, except one has a rather long article, and other has a condensed version of same article, there will be a disproportionate number of "unsubscribes" from first, and few, if any, from second. This has got to tell us all something.
The magic number, we have found, is 750 words or less.
Personally, I try to keep all articles I write (and publish in our Newsletters) under that magic number. If I let my fingers get away from me, and have exceeded that magic number, I take a long hard look at article. Nine times out of ten, it can either be shortened or made into two articles.
Most modern word processors have a word count feature. As you are developing your article, if you check this on a regular basis, you can achieve desired size without having to go back and rewrite it.
The second, and equally important "trick" is to word wrap at 65 characters per line. If you don't, people may receive your article in a format that is very difficult to read. Many mail packages default to 65 characters per line, while others default to a higher number.