The scale on such a cluster would be obscene. If they were putting 4 of them in 2U units, that would be 96 per cabinet. Let's use 100 per. That's 10000 cabinets for 1 million GPUs. Each cabinet is roughly 4 sq feet. You need enough room to walk around, which is going to be roughly 6 sq feet per cabinet, so make it 10 sq feet per. That's 1 million square feet. That's roughly a US Football field long and wide, with infrastructure, that's about the size of an enclosed football stadium such as the Louisiana Superdome, though nowhere near as tall.
With the heat that these things can produce, it's possible that they only get half that density and double the size needs.