Because this clothing store is for kids.
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.
how much space do you need for dressing?