A large coil like a 10 to 12 inch coil is good for covering a large area. 12 inches and larger are good for Cache and bottle dump hunting. If you are hunting a known old coin spot then the 4 to 6 inch coil would do fine. Most detectors come with a standard coil around 8 to 9 inches....not too small but not too big so it can do relatively well in most of the aspects of Treasure Hunting. I like the 6 to 8 inch coil's the best in my normal hunting situations....Joe