I just finished a COMPLETE set of .64 a month or so ago. Took 12 1/2 700 meg cd's = 8.55 Gig
This included, ALL roms, ALL chd files, plus verified archives of the sounds, artwork (bezels and backgrounds), marquees, control panels, cabinets, icons, screenshots, title screens), plus ALL known non-mac versions of the executables, front ends (including Emulaxian) and ad-ins, Linux boot images, AMD, P4 cross-compiles, etc.. Yeah, virtutally everything available.
Of course, now .65 and .66 are out, but at least no new .chd's were added. I would expect it's just under 9 gig for the "world".