Finally, there are cases in which you are not addressing an author's argument but are merely providing an interpretation of facts presented in a purely journalistic article. But the point remains that you must identify the essential point of the article in order to know when each of these approaches is valid. Stick to the essentials. The average length of letters printed in national newsmagazines is about 80 words. Major newspapers often print letters up to 140 words.
Student newspapers are generally more accommodating, often printing letters hundreds of words long.
But letters to the editor are, by their nature, very short. They are, after all, a response to another person's article, not whole articles in themselves. In fact, the more succinctly you make your point, the better chance it has of being read and remembered. A friend of mine recalls one of the most effective letters to the editor he has ever seen. It was published in a major newsmagazine in 1987, shortly after the Soviet Union shot down a Korean passenger jet that accidentally flew into Soviet airspace.