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.
Another way you can go is to create your own templates. You can base these around the types of articles you enjoy or you can visit other directories to see if you can find a general idea that gives you the ability to come up with your own template. Using your own articles may help you come up with your own templates as well. If your articles are along the same lines, you should be able to come up with a number of templates. Those will give you ideas for new articles and that for sure will help you get through your writer's block.