Set Area with Displaying the four points/corners

Hi, just wonder how could i able to set area(square of 500 * 500) and knowing/displaying these 4 point. it kind of difficult on setting it as the view point will keep on move (maybe at first is at (0,0)) with the movement the view point might increase so i hope that there is aniway to write the area and with these points keeps changing.
Note: the viewpoint is able to move forward and backward , turning left and right and so the point of the four corners might change everytime.*