"Fut" is part of the passé simple, which in modern French is only used in literature. "Était" is part of the imperfect tense, an action that was not completed in one fell swoop, but took place over time.
There are a lot of past tenses in French; the three you should know are the perfect, "passé composé", the imperfect "imparfait", and less importantly, unless you intend to read lots of literature, the simple past, or literary past, "passé simple".
So passé sample:
Il fut un homme grand aux yeux verts.
Il était un homme grand aux yeux verts.
Il est devenu un homme grand aux yeux verts.