Size—and cooking method—matter
If you're looking for a healthy breakfast, you can't really go wrong with an egg. That's because egg nutrition is hard to beat. It's a complete protein that's dense with nutrients. But how many calories are in an egg? According to the American Egg Board, a single, large egg has 70 calories. The number of calories in an egg can vary, though, depending on the size of the egg. And yes, the larger the egg, the more calories it has. According to the United States Department of Agriculture's Food Composition Databases, a raw, large egg actually has 72 calories. A raw small egg, on the other hand, has only 54 calories, and a raw jumbo egg has 90 calories. (FYI: These definitions of egg sizes—like "large," "jumbo," and "small"—are actually regulated by the USDA, based on weight of the egg.)
There are other factors that'll change the number of calories in an egg, though, including the way in which it's cooked. A large poached egg has 72 calories, according to the USDA, but a large hard-boiled egg has 78 calories. A large fried egg has 90 calories. A single, large scrambled egg has 91 calories, likely because of the addition of milk, and a large egg that's been cooked in an omelet has 94 calories.
Basically, if you're cooking the egg in some kind of fat, be it oil or butter, or adding milk, your egg is going to have more calories than if it was raw. And the more toppings you add to your omelet or scramble, like cheese or vegetables or sausage, the more calories it will have. But if you're eating a whole egg, you're still having a fairly healthy, low-calorie breakfast—even if you baste your egg in oil while you're frying it to get those super crispy edges.
For instance, if you eat two large hard-boiled eggs for breakfast, you're only consuming 156 calories. That's about as many calories as a cup of cooked oatmeal without any sugar; if you add any toppings to that oatmeal, however, like berries or nuts, the calorie count will jump drastically. A commercially prepared medium blueberry muffin has nearly three times the number of calories as two hard-boiled eggs: 424, according to the USDA.
Now, it's important to remember that the number of calories doesn't correlate with a food's healthiness. (I mean, a stick of gum is only 11 calories, but it's not exactly nutrient-rich.) But if you are looking for a low-calorie breakfast food that's still filling and dense with nutrients, you certainly could do much worse than a couple of large eggs.