"The building is small."
Translation:Az épület kicsi.
You never use "van/vannak" when saying what something is or how it is.
If you saying, "this is a building," it's just "Ez épület." If you're saying "this building is small," that's "Ez az épület kicsi."
You only use "van/vannk" in the third person if it's modified by an adverb or question.
Hol van az épület?
Az épület itt van.
afaik, kis never comes after the noun, while kis and kicsi are interchangeable in front of it, but there kis is more usual.
Az epulet kicsi. The bulding is small. Az kis epulet. = A kicsi epulet. = The small building.