The large buildings can only be placed on large building plots.  You will not initially start with a large building, so you will need to add those yourself.

The known large buildings: (Click for more details)

• Academy
• Armory
• Barracks
• Infirmary
• Mage Tower
• Stable
• Trading Post

While the Trading Post was not included in the initial announcement, it was shown in the UI for Large Buildings at Blizzcon.