Baresi and Passarella were small defenders by modern standards. In fact, during most of the 20th century, many CB's would be around 5'9-5'10. The biggest changes since were:
1) Inevitable human evolution - People are generally becoming taller. Gene pooling is more pronounced in sports.
2) The transition to the 4-3-3 and playing a single striker led to a gluttony of tall and strong strikers, capable of playing with limited support due to their athletic abilities (Drogba comes to mind)
So it's more just an evolution of the game rather than the case whereby you can't play CB if you're X height. If anything, the normalisation of false 9's has probably led to a renaissance of the 'smaller' CB and nowadays speed in a defence is a hot commodity, which is much more pronounced in smaller CB's.