The size of a coil can influence the detection depth and sensitivity of a detector.
The larger the search coil, the deeper it can detect larger metal items. However, it is more difficult to use in trashy areas with less depth to find small metal items.
The smaller the diameter, the more sensitive it becomes, but with reduced detection depth. Smaller coils will also be lighter in weight, easier to control and may be chosen for their ability to negotiate difficult terrain or undergrowth. Smaller coils also have an advantage in areas of high trash, due to their smaller detection footprint.