Unlike "to be" (is, are, been ...) in English which are used to describe both action and feature of things, such as, "I'm swimming" or "I'm tall", we use different words.
And "đang" is used for present continuous tense, namely, you ARE doing something.
To describe a feature, you don't need any supportive words. Just "tôi cao" - I tall, literally.
And the second part. I think you mean "đứa con gái" - "a/ the girl". "đứa" is literally translated to a kid. You can use it independently, for example "đứa này béo" - the kid is fat. Also you can use it as an article before "con gái" - girl or "con trai".