"For instance, you could say "ta shi hen gao de," for "he is very tall." This is grammatically correct, but then the sentence has a tone that is like passive voice in English."
I would interpret such a Mandarin sentence to come from a discourse context in which the person's height has already become a topic, as for example when the speaker is conceding to another speaker that the person is tall (but perhaps unskilled in basketball, nonetheless).
The normal way of expressing "he is tall," "the language learner is smart," etc., is to use what linguists often call "stative verbs" (English-speaking learners often call them "adjectives") without any use of "是 [be]."