The target size in finding the maximum point blank range is usally the vital zone size of the animal you will be hunting.
One description of the MPBR concept that has been used a lot over the years is to visualize shooting with your barrel centered inside an imaginary pipe. The size of the target determines the size of the pipe. The maximum point blank range is the furthest distance at which a bullet fired from a rifle will stay inside the pipe without hitting the top or the bottom.