Like Monte says, the 7" concentric is the best size choice.
Large coils are not always the best to use.
Any heavier coil than the stock 11" will put a whole lot of strain on your wrist. The weight will cause the detector to be coil heavy. To balance the detector you would need a heavy counterbalance weight placed under the armcuff. Thus causing another issue of pendulum effect, at the end of the swing, the coil will still want to go in that direction and will take extra arm effort to stop the coil and reverse the swing. Talk about being sore after awhile. Unless your a muscle bound detectorist. Hey, there's a great spot for one under the armcuff.
A lightweight 7" coil under such a lightweight detector design would balance just about perfect, it will feel about half the weight it actually is.
Attached are three pictures for balance to make a point.
The first detector similar to Simplex concept. It balances properly.
Second picture is the Kruzer, coil heavy. Will take upwards wrist pressure to hold in a balanced position.
Third picture, Kruzer with optional battery pack installed as a counterweight. Better but, still not balanced.
Now imagine a larger than stock Simplex coil mounted on it.
The lighter 7" coil makes a big difference in overall handling and weight distribution.
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I am all for using a lightweight balanced detector.............