You can have 12 cups of Orville Redenbacher's Smart Pop popcorn (2 bags). That would be 200 calories, 4 grams of fat and 6 grams of fiber.
Making popcorn at home? Keep in mind any oil used in popping it. Each tablespoon of butter you add to the top is 100 calories and 11 grams of fat.
Theater popcorn is trickier. It can range from 400-1200 calories. A large popcorn can be 16-20 cups with 1-3 days worth of saturated fat. Most popcorn at the movies (even a small size) will be more than one serving size.