The two xbox360 japanese rpg's already came with multiple dvd's (Lost Odyssey (4 discs) and Blue Dragon (3 discs)) , Final Fantasies being heavy on FMV and long games with very diverse environments, I think it's safe to assume FFXIII won't fit on single DVD.Fixed
@radidrabbit: what makes you think FFXIII will be so especially demanding it will need an install? JRPGs are the last kind of game I'd expect that in. FWIW the only games requiring an install on 360 IIRC are FFXI (an MMO) and that football manager.
Of course multiple discs doesn't necessitate installing, I'm just assuming
Anyway, FFVII being on four discs didin't really kill the experience for me in 1997, so maybe disc swapping in a linear rpg in 2010-2011 isn't that bad after all.