A good rule of thumb I often see bounced around is an 1" for every 10 yards. So0
10 yards= 1" grouping
20 yards= 2" grouping
30 yards= 3" grouping
That covers the ranges most people shoot at whitetails. If you can consistently keep it within these groups and aim for the right spot to give you the most margin for error in the kill zone, you'll kill alot of deer, novice or pro...
Up to those points, the kill zone on an average whitetail is about 9", so in reality a common pie tin will do, provided you aim correctly. However, that doesn't leave one much room for error, and you should practice until you can at a minimum shoot the above groupings.