Even at papers that don't have a word limit, shorter letters will get printed more often than long ones. More importantly, short letters will attract more readers. Most papers won't print long letters, personal attacks on non-public figures, or theological arguments. In addition, your opinion will be much more effectively expressed if you back up your points with evidence and keep your tone calm. Sign it and if possible include a phone number for verification.
Every little bit helps.
Here's a real-life sample of how this might be done. A friend was amused by a report in the Chicago Tribune of a church in nearby Evanston, who objected to Halloween celebrations in public schools because they allegedly taught witchcraft and Druidism. He could take the matter up directly with the church, but that wouldn't likely get him very far, because it doesn't sound like this is a group that's open to having its beliefs challenged. But a letter to the editor could open some eyes, if written with intelligence and perspective. I would certainly object if anyone used my children's educational time to teach Christianity or any other faith.