One of the most commonly used vegetables in the United States, eggplants are well known for their elongated shape and purple color. Usually 4 to 9 inches long and 2 to 4 inches wide with a dark purple skin, they should have a green calyx or stem, and the skin should feel firm to the touch and be smooth and free of wrinkles. Eggplants are a great source of dietary fiber, potassium and manganese.