The closer they are the smaller the number shot you can get away with. However, they seem to be pretty tough birds to me, so I would tend toward #6 or even #4 if you are specifically purchasing for the purpose, and the cost is not prohibitive. I have used #7 with luck, but you have to be patient, and cut loose at the last second before they land in your lap.
Most of the fun is in the missing and laughing at each other. It is not like you get a good meal out of it or anything.