According to recent mapping of the Great Wall of China, the wall and its various extenstions is over 8850 km long. Still, even with the added portions from the Ming dynasty, this wall is not visible from the moon or from space (by human eyes only) as any tour guide will tell you. The wall(s) is/are indeed a massive undertaking but I would not put it on the seven wonders of the world list for three reasons. (1) it was constructed during almost two millennia (and the portions you as a tourist sees are the remains of heavy reconstructions only a few decades old). (2) a wall is not an architectural difficulty. (3) it may not even be the largest “single” human construction. I believe someone has argued that the Roman road network had much more labour invested in its construction (and what about our modern roads?). I would rather put the Temple of Heaven on the seven wonders of the world list.