It means Dora is six years old, and remarkably poised for being so young.
When we say someone is [adjective] for a [trait], we mean that you wouldn't expect a standard person exhibiting that trait to embody that adjective.
What may be confusing you here is that it also contains ellipsis of the word "being" and the elaboration on the age; the full sentence would be
"Nice to meet you," said Ray's daughter, Dora, who addressed me in English and seemed remarkably poised for [being] six [years old].
Omission of words in a phrase can be a useful way of conveying information more concisely when the context is clear. In this case, the writer seems to be relying on the reader knowing that the only logical interpretation of the word "six" here is a reference to Dora's age, but in this case it seems to have resulted in an ambiguity where you thought perhaps "for six" was an idiomatic phrase you hadn't encountered before, similar to its use in "knocked for six" which means to be considerably shocked or upset.