This post will address size, not kind of scoop. I recommend trying to get a scoop that is comfortable and as large as your coil. So if you have a 10'' coil, a 5'' inch scoop is going to make it difficult to pinpoint and cause you to scoop multiple times. Thus if you have a 10'' inch coil you should probably get a 9-10'' scoop. I have a 9'' scoop for my 10'' coils and it works very well. I think you could probably get away with a 7'' but that is on the way lower end. 6'' would be too small.
Now for travel, I have a 5'' only because it needs to fit into my car but for home digging I have a scoop that is as large as I could find.