"The room is wide."
My best guess is that in this case it is a specific statement. When statements are specific 이/가 are more often used, whereas general statements use 은/는.
If the sentence was "Rooms are wide", 은/는 would have been appropriate.
I am struggling with this as well on Duolingo, since the lack of context makes it hard
maybe because -어요 is less polite than duo expects here? Up til now it has always been the deferral form.