* μεγάλη κοπέλα (megali kopela): This translates literally to "big girl." It's straightforward but might sound a bit childish or even slightly derogatory depending on context and tone. "Κοπέλα" (kopela) is more suitable for a younger woman.
* μεγάλη γυναίκα (megali gynaika): This means "big woman." "Γυναίκα" (gynaika) is for a grown woman. While accurate in terms of size, it doesn't specifically imply youthfulness like "girl."
* γερά κορίτσι (gera koritsi): This translates to "grown-up girl" or "mature girl." It suggests a girl who is older and more developed than a child, but still relatively young.
The best choice depends heavily on the context. If you can provide more information about how you intend to use the word, a more precise translation can be offered.