An 18650 battery is 18mm in diameter, 65mm in height and has a cylindrical (o) shape (hence the numbers 18-65-0). They might be large but they're one of the most popular types because they're powerful and long-lasting. Other sizes you may find in some e-cigarettes include 26650, 20700 and 21700. It is essential that consumers always use the right battery for their kit.