From my experience. On a PI, larger coil greater depth, harder to pinpoint a target, more susceptible to EMI emissions.
8" coil almost the same depth depending upon circuit design, much easier to pinpoint. Less drag in the water which makes it easier to use.
You will find most guys using the Headhunter PI prefer the smaller coil. I have been using the 10.5" water hunting and just built a 9.5 for water hunting.
And use a 7" for hunting trashy areas on land.
The coil size of most PI's normally ranges from an 8" diameter coil to greater than 3 feet in diameter. It is quite common to hear of a person using an 18" diameter coil, but the most popular sizes range from 11" to about 14".
As one might expect, the larger the coil, the deeper one may find objects. However, it is quite possible a smaller coil will find an object deeper than a large coil, especially if the object is small.
.......there is a more direct relationship between the size of the coil, size of the object and the ideal maximum depth such an object can be detected.When searching for information about depth or size of objects that can be found with different size coils, extreme cases always show up. For example, many people have found extremely small nuggets ranging in the few grain range with an 18" coil. Normally such a large coil will not be able to see such a small target at any depth, or even in the middle of the coil if the nugget is small enough. However, this small nugget can produce a signal if it is very near the coil windings themselves. ...............a smaller coil will not show as dramatic increase to sensitivity to small gold on a PI like it does on a VLF. The reason, again, lies in the fact that the sensitivity to small gold on a PI, is much more dependent upon the delay before sampling than it is on the coil itself.