Is not about the method is about the material.
Porcelain Material can be used for bridges no longer than 3 units. A Porcelain Block for Cerec milling machines can make crowns and bridges no longer than 3 or 4 units as the blocks used are about 2 inches long, that is the aproximate size of a 4 unit bridge.
If it is made with zircon it wont matter the size, but Cerec Milling machines does not provide very good anatomy as the burs used by Cerec are too big for deliver propper anatothical details. Alternative materials that can be used are Bruxzir or Zirlux.
Cerec from Sirona is very difficult to see on this days (very old technology) as there are other brands and milling machines that are superior to cerec in many aspects.