18350 batteries are the most compact battery currently available. 18350 batteries are much lower in capacity when compared to the more common 18650 size, but its smaller size allows you to keep your device as portable as possible while still being able to replace the battery when you need to.
Some notable compatible devices are the DotStick and Dot AIO Mini.
Always check the product page to see if your device needs batteries, and if so which size is compatible.