Because newsletter templates are available in all variety of lengths and sizes, there is really no limit to the content that you could potentially write. Keep in mind, however, that you should probably keep down your word count to a level that people are comfortable reading. While you may think everything that you are spouting off is fascinating, it's helpful to first sit down and make an outline of what you want your main message to be, or what the major news is that you are trying to share.
I'd like to give you an example: I recently wrote an article outlining several ways to get out of a writing slump. As I was writing that article, I came up with about 7 or so ideas for getting your brain out of a creative rut with your articles, including some article template ideas and some writing productivity techniques. I submitted the article, and then thought__ "Each of those ideas for getting out of a writing slump needs to be elaborated on and explained further. I can do a new article for each idea." And so I did__so far I've bounced 4 or 5 articles off of that first article, and I have even more ideas that I've gotten from that first article that I haven't written about yet.