A large search coil only makes a difference when you are searching for targets buried deeper in open areas, by open areas I mean areas with little to no iron.
Put iron in the equation and it does not matter how much non Ferrous stuff is in the ground you probably won't detect it.
The last time I ran across a productive site with multiple good targets I was using an 11 inch search coil.
Because the area was littered with iron, I went smaller instead of larger and found more good stuff.
I know from experience that a larger search coil would not have detected any more good targets.
A larger search coil would have been one big null, even with the removal of multiple non Ferrous targets from the area.
Instead, I was able to cut down on iron masking and detect more good targets.
A perfect example of bigger not always better better, and showing how target depth is not everything to a beach hunter.
I still search areas that I see other beach hunters walk right over without detecting anything using 11 and 10 inch size search coils.
Sweep a smaller search coil over the same area and out pop small gold bands, ear rings and chains.
You don't have to always go big, the hardest targets to detect are often the shallowest targets.
A recent bucket list find I recovered popped out of a 3 or 4 inch deep hole, it just needed to be detected.
As a guide, use larger search coils at beaches with little to no iron, taking advantage of the large coils best features, ground coverage and target depth.
Use smaller coils at trashier beach sites, taking advantage of the small search coils sensitivity to small targets and enhanced target recovery speed between the trash.