Unlike the English word "big", when applied to people, groß does NOT mean "fat" but it means tall.
In reality, it's mainly used with children, because a tall person will usually know that they are really tall and don't need that pointing out to them, nor is it a compliment as such. Kids, on the other hand, are flattered by it, coz they usually can't wait to be "grown ups". Little do they know that it's not all that it's cracked up to be...
I seems to me that is should be accepted as a possible answer. Granted, it is usually used in the "she is so tall" sense but one can easily imagine someone using big in the same sense ("I haven't seen your kids in ages! They are so big now!") . Additionally, one can use that sentence while not necessarily referring to humans, in which case "tall" would not be appropriate ("So this is Miss Fluffy? She is so big!"). That the translation need not be the most obvious one should be especially considered in light of some more abstract practice sentences one can find on duolingo.
I think big shoud be accepted. I am from mexico, we also use big to tell somebody he or she is big. If you use big in the USA, you get people here and there offended, but I believe Duolingo should understand that there are far more international learners than just the "american" ones.