Only 1x1, room for 1000. As my storage is over 1 mill, i dont really care about 1000 extra storage. BUT with such a small footprint it can be placed in places the normal storehouses cannot (not to mention improved storehouses).
The only problem is - workers dont go there!
This is zone 4 with the 2 iron mines and the little warehouse tribute next to it
http://prntscr.com/ov8c0g
-the workers from the iron mine still prefers the long walk to a more distant storehouse.
So whats the point with the warehouse tribute?