Size of the building

As they approached a wide central plaza, a massive structure of pastel blue crystal loomed over them.  The building was shaped like a mushroom, with a wide rounded cap resting on top of a round crystal shaft at least three miles in diameter.  The crown of the building--at least five miles across--had long, irregular shafts of crystal projecting from it at many angles.  The central structure was decorated with weird modernist statues that looked to the explorers like the wildest examples of abstract art.