"여자는 키가 큽니다."
Translation:The woman is tall.
Literally: “The woman… height is big.”
는 is the topic marker. I'm this case, the topic is the woman.
가 is the subject marker. The subject here is height.
This sentence is stating something about the height in relation to the woman. So while 는/은 and 가/이 are used interchangeably in some cases, this is one where they are used with their finer nuances.