Coil size matters in metal detectors because coils are the hardware component of a metal detector which enables the device to through signals underneath the ground. The larger the size of the coil, the more the depth it covers underground. Thus it could be said that small coil covers less area. But what is benefit of small coil? Small coil is used for accuracy. The small pieces of metals could be found with accuracy only. For example if you are working with large coils, then you can miss small objects. Going on deeper levels compromises sensitivity of the device. By sensitivity, we mean what is the minimum size of target object that could be found.
Coil size matters in metal detectors
In addition to coil sizes, coil shapes also matter. The basic coil shapes are a mono coil, a DD coil and a concentric coil. A mono coil is only a round loop of coil. It works best for better depth with no discrimination. A DD coil has loops shaped in double D pattern. It works best for discrimination. But you have to compromise on depth. The concentric coil has a circular loop within the loop of coil. It is best for more coverage of area underground. Moreover, manufacturers are consistently improving designs. For example you can see elliptical coils for better accuracy and precision.
What is the best coil? One of the best combination in the market is a Mono Coil paired with a DD coil. So you can get the best of both worlds.