The largest machine in the top500 draws 30MW, which is getting to be close to the size of a Facebook or Google datacenter. All the rest are much smaller. Mostly people misunderstand the relationship between supercomputers and the cloud. Supercomputers are somewhat large and very specialized. Cloud datacenters are just enormous.
Many years back the military connected some 1700 PS3s to create the world's 35th most powerful supercomputer. That needed few racks and could do 500 tflops. One latest XBox can do 12 tflops sitting in your living room. Of course the supercomputers would also have gotten magnitudes faster since. But hope this gives some sense of physical size.
