To answer your question as asked: 6" coil. However, I typically hunt in wooded areas and need the small coil to get around trees and downed limbs.
When hunting a field or park, well, it depends on the amount of trash. It's awfully difficult to separate (pinpoint) targets in a trashy area using a 15" coil. In these areas I use as large of a coil as the trash allows.
Stock 11” coil is almost always on my 800 for both parks and fields. I prefer the greater coverage area provided by the 11” compared to the 6”.
I’ve owned the 6” coil for as long as I’ve owned the machine (about 1.5 years), but I can count on my two hands the number of times I’ve used it. I usually only strap it on when corn or bean stubble in harvested fields interferes with an effective swing with the larger coil. My overall impression is that I tend to get fooled by iron nails a lot more often with the 6” coil.
I don’t own the 15”, and I doubt I will ever spring for a new one (just too darned expensive). Might pull the trigger if a used one pops up.
I prefer the stock coil on the 800. I agree with Airmet Tango the 6 does seem to call iron a good target more often and I use it only where the larger 11 in. is hampered. The 15 I have, Its too heavy for me and in my Penna. soil I don't see much of an advantage over the stock coil. I think Minelab got it right with a good match of machine to coil. Good luck Mark