I haven't given it to her yet, so I'm not sure if it will fit her. I made the size 4/5 and she is turning four, so it will probably have some room for her to grow. I did try it on my three year old son
and it was way big for him. The sleeves seemed really long! I hope it fits my niece, but we'll see!
It was fairly quick to knit up- you should definitely make one for your great-niece!