How much disk space is already used by the OS and included apps on a new MacBook Air
I have a 128GB Air with Lion and iLife '11 ... which is what, by default, would come on a new Air from the Apple Store or authorized reseller. I have other apps, so this is only a guess, but...


Lion: 4-5 GB 
iLife '11: 5 GB 
Recovery Partition: 650 MB


So, on a 128GB Air, you should see about 115-118GB free space. On a 256GB Air, you would see about 243-246GB free space.