There are several Mexican dishes which contain a small number of calories. If you wish to keep your calorie intake to a minimum, you can choose from the following dishes: 1) Chips and dip because 20 chips or 20 ounces of chips contain 300 calories, 1 ounce of guacamole contains 108 calories, 2 tablespoons of salsa contain 12 calories and ½ cup of cheese dip contains 246 calories, 2) A large beef burrito contains 424 calories, 3) Chicken chalupa contains 400 calories, 4) A medium beef burrito contains 272 calories, and 5) Ceviche. It pays to nutrition calculator if you really wish to check how much calories one Mexican dish contains.
Romain lettuce only contains 5 calories, so I guess this is the Mexican food/ingredient with the smallest number of calories.