"That tree is that tall."
Translation:그 나무는 그만큼 높습니다.
I understand that, from what other people said, you can't use 키가 크다 unless you're talking about people, but my question is, if it were acceptable to use it to talk about inanimate objects, wouldn't the sentence have to be "저 나무는 키가 그만큼 큽니다" because "저 나무가 키가 그만큼 큽니다" would have two subjects?
"높다" is the dictionary verb stem meaning "to be high/tall", which can be conversationally conjugated as follows:
존댓말 격식 / Polite formal - 높습니다
존댓말 비격식 / Polite informal - 높아요
반말 / Casual - 높아
All three should be accepted, but considering this is in beta, they may not have been added to the accepted answers list. Please report such findings in future to help develop the course :).