The name 18650 means that the battery has a diameter of 18mm and is 65mm long, but when a protection circuit is added the battery will be a few mm longer. How much longer will depend on the actual protection circuit and how it is mounted.
These round high capacity cells have been mainly used in flashlight type applications but with its capability to be used as a drop-in rechargeable cell at 3.7V with a capacity of 2600mAh. This is a great battery option for those of you who need a simple to install and replace cell with a lot of juice.

These 18650 Cells have a standard discharge current of 0.2C to a maximum of 1C and can handle about 300 charge cycles.