Gears of War was 6.43GB
I think its highly unlikely that GTA4 will be bigger then 8.5GBs.
I have seen the Final Fantasy XIII trailers and I still think at most we'll see it able to fit on 2 Dual Layer DVDs (17GBs) and again whats wrong with swapping discs? we did it with so many other RPGs before, have we become so lazy that we can't get up off the couch for 30secs to swap out a disc? The only thing that could push it to need 25+GBs is if its full of a ton of FMVs. But I think Square is coming to an end with needing thous.
Currently playing:
Wii: MySims | Xbox360: Halo3 / Pac-Man C.E | DS: Scurge | PSP: Ghost in the Shell
"Lesbians, Better then eMail."
~Lanky Man - My Big Fat Independent Movie