If you go by a single spread. than the group size (radius or diameter) increases directly with distance. If you go from 1 inch at 20 yards then it will be 2 inch at 40 yards at 4 in at 80 yards. The area of the group increases with the square of distance. Basically to first approx sprewell and bus are correct.
.... assuming identical muzzle velocity, projectile weight, and ballistic characteristics on every shot. Which never happens.