not sure what you mean by shingle beaches and I am no beach hunter but in my opinion if I were hunting a beach I would use the largest coil I could possibly swing which is about an 11 inch coil is all I can muster these days I tend to use a 9 or 10 inch by 5 inch coil any more most of the time
One more thing I was taught a long time ago
(the only dumb question is the one you do not ask)