When it gives a serving size of "3 cups" for lettuce how does that work? Because if you just toss in some lettuce in a measuring cups it won't take much to fill up 3 cups. Or is it referring to lettuce packed down? I'm just not sure how to measure it.
Shredded but not packed down. But it doesn't really matter, 3 cups of lettuce has about as many calories as a friendly kiss.
Furthermore, most lettuce packages and almost everything here on site give the weight in grams. If you are that concerned about lettuce, which is mostly water, then weigh it.
Yeah, lettuce is not even worth counting. It has about 15 calories a serving depending on the type, but after digestion it's basically nothing. You can eat as much lettuce as you want and not gain an ounce. It's the additives that you need to worry about.
If you haven't used http://nutritiondata.self.com/ you might want to give it a try. Some items, such as romaine lettuce, can be calculated by leaf (inner vs. outer). It's a great website to use if you're looking for conversions between actual food items and cup-measures.