Most periodicals (newspapers and magazines) solicit letters from readers for publication. Due to space restrictions, these typically have restrictions on length, and often the restrictions are explicitly expressed as number of words that are allowed. It was to get counts of the words in letters (or sections of letters) that I wrote Wordcount.
Here are some examples of such guidelines.
Other publications don't give explicit word counts, preferring to just suggest brevity without defining it. You can check out existing letters on the web sites of these publications, and use Wordcount to see how long these are.