I only use 500gb ones in my machines, but I keep of backup collection of roms that is on a 1.5TB drive with very little room leftover.
That includes a full MAME romset+CHDs, Daphne, Taito Type X, Demul, Makaron, Model 3, Model 2, Digital Leisure light gun games, all cartridge based systems, US release Sega CD, US release Sega Saturn, & Dreamcast. If I have video snaps on there, they would only be for arcade games.
The consoles that had optical drives are what really eats up the space. I only have isos for the sytems listed above, no PS1, PS2, NeoGeoCD, TurboGrafx CD, GameCube, Wii, etc. I once saw someone selling 2TB drives with that they claimed was the entire PS2 library.
2TB is probably the going size for pokerom types (gotta catch 'em all) and they limit the optical images to the most popular titles or the first few consoles of that type.
I can see hitting around 6GB if you wanted everything that is out there. The PS2 alone is more content than you could play in a lifetime.
It would be interesting to post actual numbers for each system.
Someone from the Hyperspin team should be able to tell you what size the total content they have available for download is.
Likewise with EmuMovies.