There was one site I found that claimed 14-16" for deer, but I think that's quite optimistic. Or one heck of a big deer!
Or a typo
6 inches seems to be recommended most often, but it really depends on how you look at it. Sure - the lungs/heart may be a 6 inch zone - but as mentioned some people consider the shoulder / spine a viable kill zone too. In which case you extend the size of the zone vertically considerably (tho not horizontally.)
Depends a little on the gun too i think. 2 years ago, my dad shot a moose and struck between the shoulder and the spine. The spine was intact, the shoulder was intact. However, he was using a 300 wthby with 200 grain tbbc's at a range of 100 yards +/- . The animal dropped in it's tracks. The damage to the spine from the shockwave of the bullet was considerable, even tho it was not struck. For a full TWO FEET along the spine there was severe bruising and indications the spine had been displaced by the shockwave of the bullet - the wound was NOT immediately fatal but it was instantly crippling. The moose was quickly dispatched with a second shot.
Clearly, the power of the gun is such that even a near miss on the spinal column will produce instant paralysis and severe CNS damage.
Would the results have been the same with a 30-30 using 170 grain bullets? Maybe. there's no doubt the damage to the spinal column would have been less severe. Now i haven't used a 30-30 on moose or even seen one shot with a 30-30, so i don't know, but possibly that shot would be a little more 'risky' with that gun than a 'traditional' bread-basket shot.
So - if we were to assume a 30-30 shooter would be 'playing it safe' by only targeting heart and lungs, he'd have a smaller 'target' than a guy shooting a bigger gun who could afford to consider breaking both shoulders or for whom a 'cns' shot is more practical as a 'sure shot'.
And of course, it depends on the angle as mentioned. the 'kill zones' mentioned assume a near-perfect broadside. The moment the animal begins quartering to or away, things change.
That's where i think some of these 'kill zone' sizes come from - on average on a moose you probably have at least 8 inches to play with, so if you know you can drop it into that size a zone, you're covered for a wide variety of angles/shots/guns, etc. Nicking the top of one lung will still likely result in a dead animal, but it could get a lot farther than if you get the ole' double lung and destroy most of the lung tissue.