Why is the fitting room "so" small? -> 탈의실은 왜 "이렇게/저렇게/그렇게" 작아요? Ought to be accepted in my opinion.
You are correct that is a perfectly natural way to say it in Korean. Actually I hear 이렇게/그렇게 quite often here.
It shouldnt. 이렇게/그렇게/저렇게 means like this/like that/like that respectively. 너무 means so/too, and most recently it has been accepted in the dictionary to mean very.