Are you the girl or are you purchasing for her? I am going to assume that you are buying for her. So, what phone does she have currently? Big one or small? What features does she want her phone to have? Couldn't you bring it up in casual conversation and then you would have an answer.
I am a woman with small hands and I hate small phones. Yes, I have to mostly use my 7+ with both hands but I wouldn't have it any other way.
Also, since all girls are different I'm not sure anyone here can truly answer this question for you other than her.