Coil selection is all about location and style of hunting.
I have all three coil sizes. In general, I start searching an area with the largest coil I can swing without obstruction, regardless of the amount of trash. At first I'm trying to cherry-pick the easy finds as fast as possible. Then I go to the smaller coils as necessary.
It turns out that the 6" is my most used coil. Relic hunting is my game and the places I hunt are typically iron infested and/or heavily wooded. The 6" coil does an excellent job of getting into tight areas with brush, saplings, etc. lt also minimizes the number of targets beneath the coil, which I believe helps separate the good from the bad. The fact that the soil here is mostly clay helps in that targets typically are 3" or less. Also, the 6" coil pinpoints like a boss.
