Bananas originated in Malaysia, but as of 2010 they are most commonly found in the Caribbean, Central America and the Canary Islands. They are packed with vitamins and contain around 100 calories each. Calories in Bananas A medium-sized banana measuring between 7 and 7 7/8 inches long contains 105 calories, the FatSecret website calculates. Smaller bananas, between 6 and 6 7/8 inches long, pack 90 calories, while larger ones, between 8 and 8 7/8 inches long, contain 121 calories. A cup of mashed banana yields 200 calories. Nutritional Information A medium-sized banana represents 5 percent of your daily calorie intake. It will provide you with 0.39 grams of fat, 26.95 g of carbohydrates, 3.1 g of fiber, 14.43 g of sugar and 1.29 g of protein, FatSecret reports. Potassium According to the Banana.com website, doctors recommend bananas to patients with low levels of potassium. A very large banana, one measuring 9 inches, contains as much as 602 milligrams of potassium. Potassium helps to control the fluid balance within the body and may help to lower the blood pressure, the United Kingdom's Food Standards Agency reports.