Props for going the extra mile for science!
While at it, we may as well complain that the 5 foot iPad models on display arent real.
Helium has a lifting force of about 1 gram per liter. (source)
A typical amusement park balloon holds 14 liters of Helium.
The 11in MBA weighs 1.06Kg (1060g), so you'd need about ~76 balloons.
The 13in MBA weighs 1.32Kg (1320g), so you'd need about ~95 balloons.
This is assuming the balloon itself and string are negligible in weight... so you may wanna throw in some extra balloons
if the balloon was real would it really be able to lift that air?
It's exactly so, for further reading one can read this - http://cr4.globalspec.com/thread/21168/How-much-helium-do-one-need-to-lift-1KgSince the Air weighs 1.06Kg and as another poster mentioned helium has a lifting force of 1gram/Liter, it's a very straight-forward bit of math. 1060grams=1060 Liters of Helium (plus change for balloon and string mass). That is equal to a cube of about 1.02 meters on all sides, or a spherical balloon 1.27 meters in diameter.
It probably could fit in a window display, but I think the effect would be lost.