"Is she big?"
I have come to the conclusion that this exercise wants to teach us that when asking "Is he/she/it big?", you can simply say "Je veliký?" and "Je veliká?" (masculine and feminine), but for neuter you have to include the particle: "Je TO veliké?". Is that correct? And is this also valid for any other adjectives, or only for "veliký"?
Yes, you can also say, "Je veliké" (but not here, because we know we need the feminine form of the adjective). Please read the earlier discussion regarding the use of a pronoun in sentences like this. In answer to your last question, it would apply to the use of other adjectives as well.
Thank you for your reply! I did check the earlier comments, but could not find anything regarding my specific question. Just to make sure: for the neuter phrase, the "to" is NOT compulsive? Because that was my impression from these Je veliký/Je veliká/Je to veliké exercises.