Pretty sure they are 18650, the most common size.
You really need to know the make/model of your batteries. It's usually the first question asked on these forums, because battery safety is paramount. As you can see below, there are a lot of green batteries.
Battery Basics for Mods: The Definative Battery Guide for Vaping
- A popular and essential read to understand which batteries are safe to use in mechanical and regulated mods. Includes a frequently updated list of recommended safe-chemistry, high-drain batteries with their specifications.
You should also know what resistance your coil is off the top of your lips. That's the second question usually asked.