Fruit Serving Size

Ok, so a banana is considered two servings of fruit, not one. Is this number of calories, nutritional, or other thing that determines the size of servings?

It is a number of things. A large banana is 2 servings, but a small portion can be a banana, an apple the size of a tennis ball, a portion, but a large apple (the size of baseball) would be two servings. The most accurate way is to buy a ladder and get an online calorie counter. You can buy an electronic balance for about $ 20 that weighs in ounces and grams and many calorie counters actually give weight and calorie fruit.