I've used most of the available coils for my detectors at one time or another. As I often hunt trashy parks and tot lots, I've found that the smaller coils are my best choice as they handle trash well and I don't need large coverage or great depth, usually. As mentioned above, large coils do cover more ground and get a bit more depth, but they are heavy and can drive you crazy in trashy areas. Where an 8" coil would be considered "stock" I often hunt with a 7" which is a pretty good compromise. Sometimes a 5.75 or even a 4 if the trash is extremely heavy. Every now and then I hunt large, clean areas where I can make good use of the larger coils, but I find them to be heavy and tiresome after a bit and generall end up using the stock in many of those locations as well. IMHO, for most coin shooting etc., one will be better served by using coils on the smaller side versus the large ones. They pinpoint more closely as well.
My two bits,
BB