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.
Remind them why they need you. Your proposal should immediately remind the prospect of what their problem is and why they are talking to you in the first place. In other words, you should immediately start your proposal off with the pain your prospect is feeling. Are they not making enough sales? Is their message not getting out to their audience? Could their life_savings be in jeopardy? They are talking to you for a reason.