"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.