Needles are measured tip to tip when attached to the needle.
Most circular needles can't include a cable short enough to create a 16" due to the length of the needles themselves. Most of the circulars like Knitpicks Options have the 16" sold separately. The very short needles needed to accommodate a short cable can pretty uncomfortable to use.
As an alternative you can learn to do magic loop or use two circulars. I have the 16", but prefer to use magic loop now because you can start a circular project like a hat and finish it w/o having to switch to DPN or another circular method for the decreases.