"His home is large."
Translation:Lia hejmo estas granda.
"esti" (to be) and "flugi" (to fly) are two very different kinds of verbs.
"To fly" is a verb of action, and so you can use an adverb to describe how it was done.
"To be" is a verb of status. It links the subject to the predicate. In "Lia domo estas granda", we're describing the house.