Mi tía es alto. altos. baja. bajas. Which sentence is the correct?

Here's why:

* Alta is the feminine singular form of the adjective "alto" (tall) and is used to describe a female person.

* Altos is the masculine plural form of "alto."

* Baja is the feminine singular form of "bajo" (short) and bajas is the feminine plural.

Since you're talking about your aunt (a female), you need the feminine singular form of the adjective.