Unfortunately, it looks like it could be more than a terabyte.
Size of Earth Surface textures in SolarSystemHD & SolarSystemUltra:
[*]Layers 1-4: 129 MB
[*]Layer 5: 297 MB
[*]Layer 6: 944 MB
[*]Layer 7 (Solar System Ultra): 2,790 MB
It appears that with each new layer, the size approximately triples. Thus, to get the entire Earth's texture to the precision of the Grand Canyon mod (Layer 11, with bump at Level 12), it could take up anywhere from 200 to 700 GB.
This is the surface layer only - to get the bump map (which increases by about 350% with each step) would probably take an additional 2 TB, and that's not even taking into account the lights and the clouds.