"What a brave girl!"
Translation:Kia kuraĝa knabino!
Kia/tia is the adjectival form of the correlatives, kiel/tiel is the adverbial form. To qualify a noun the syntax requires an adjective ("kuraĝa knabino", "tia knabino"), to qualify an adjective requires an adverb ("vere kuraĝa", "tiel kuraĝa"). So if the "such" in the English sentence refers to the braveness, I think it would require the adverbial "kiel" as its translation. Strictly speaking the English sentence would have to be "So brave a girl" for that, but if "Such a brave girl" is spoken with the emphasis on "brave" I think feels idiomatically equivalent. Depending on whether you want a literal or semantic translation, you could go either way.