I know they can get absolutely huge. I remember when I was about 12 years old I went to spend the weekend at my cousins house who lives on the edge of a good amount of farmed bottom ground. This time the creek about a mile from his house was flooded and we went out on a bike ride to have a look. There were night crawlers 1 to 1 1/2 feet long all along the fields next to the flooded area trying to get to dry ground. They were big enough that my first thought was that they were snakes. I haven't seen night crawlers that big before or since.