Looking at the quick storage analysis of my MacBook Air I'm currently using 177GB out of a 256GB SSD.
I've got
- 20GB of applications (presumably this includes OS X?),
- 42GB of iTunes music,
- 43GB of iPhoto pics,
- 18GB of an iMovie project, plus
- 54GB of 'other' which includes a Parallel's Windows XP setup.
There's no way I'd expect the OS X software running the Fusion hybrid to keep my music, photos and movies on SSD. Maybe the indexing files, but not the actual music and photos.
And everything else, including a full installation of Windows XP and associated applications, is a grand total of just 74GB. I would have thought 64GB of SSD would probably work quite well, but having 128GB sounds just fine to me.