Here we need to choose an adjective that is the opposite to alta that means tall. Recall that an adjective is a word that gives us more information about a noun or pronoun. So the opposite adjective to tall is small and this translates into bajo/baja in Spanish. Finally, we can write and translate this sentence as follows:
<em>Una persona alta no es </em><em>baja </em><em>(A </em><em>tall</em><em> person is not </em><em>small</em><em>)</em>