I think there is some common sense involved. No I don't think its reasonable to say you need a mile radius, but you also need to think of a ricochet at a slight angle so it keeps a lot of velocity and can also add some height possibly going further than you intended.
Keep in mind the skill of the person shooting the gun to take those variables into account. Is a person going to keep that in mind all of the time or get excited about getting the animal and circle around and make a bad choice.
As mentioned the size of the animal does matter, but I am curious as to when a pellet gun is mentioned, is that a wally world special or a higher end break barrel model?