Take a look at MMVapors Nanos... 18650 is 83mm... or 3.3inches. Raw power in a small package. Of course if you're going for 18350... you're looking at 53mm or 2inches (uncomfortably small imho). and the 18500 is 68mm (2.7 inches). All very small, just need to decide your power needs.
Size is small, but the kicker is the lock mechanism... just a simple flick locks and unlocks. By far, my favorite locking system.