"My brother is big but my father is small."
Translation:Tá mo dheartháir mór ach tá m'athair beag.
In the simplest sentences you use is with "noun=noun" sentences, and you use tá with "noun=adjective" sentences. (that's a gross oversimplification, but it's the starting point).
"big" and "small" are adjectives in this sentence.
(It is possible to use beag as a noun, but that's not really relevant in this case).