I personally use my 6" coil the most after getting it. The six inch, for it's size, is amazing at the coverage it gives. Add the separation and it is hard to take off once it's on. I feel I don't have to overlap intensely like small coils on my other detectors.
I suspect that the 15" will be beyond the point of diminishing returns, but I am following the threads on the 15. Thing is, if the 11" stock coil is as good as it is, and the 6" finds missed stuff that the 11" skipped over, then I can't see the 15" investment, at least in my local area, for coin and jewelry.
I once read that a 13" coil size hits the limit for depth. Who knows though. The Equinox has already surpassed my expectations.
I am waiting on an 8" coil.