Those measurements make your building 16 square feet. At 3 square foot per chicken you are looking at only having 5 chickens. Therefore, I would not add any more to your flock
Don't be discouraged however. The idea is for the chickens to be healthy. If they become overcrowded they will begin to pick on each other, peck at each other, pull feathers and other undesirable traits will appear. Perhaps in time you can build a bigger building ( coop) for them.