Well, FFVII was 3 discs and it turned out pretty well.
I guess there is always the option of buying a 500 dollar Blu-Ray disc player for those that don't want to get up 3 times during their 60 hour game.
I wonder if gametrailers is gonna do a comparison video?
Cause we all know in the past, how the PS3 versions of multiplats have been far superior, with shorter loading times, better graphics, and no mandatory installs.
Blu-Ray is a talking point, the PS3 and 360 have graphics so close it's nearly impossible to tell the difference, and in general games don't need Blu-Ray. The only cases in which games are going to use enough space to warrant multiple 360 discs is when those games are designed with Blu-Ray in mind, and the developer just had fun playing with the novelty of infinite space.
If not perpetual, disc changing takes less time than mandatory installs, and hoping, assuming, or generally preying for a lower quality port of FFXIII for the 360 bears the remnance of the scar still left by that fateful hour in which all of the Sony talking points were destroyed, and reformed within the span of a single trailer.
To answer the OP: How will they fit FFXIII on the 360?
Slowly at first. Inch by inch.