At nine months she is already full grown. Quakers differ greatly in size though, some like our Casey are much smaller and some are much larger; they vary between about 80g and 150g.
As a general rule soon as a baby has fledged that's approx the size they will stay...While they will gain weight and as has been mentioned their tail feathers may become longer body wise they are about as big as they'll be. You can pretty much count on them being full grown by their first molt which usually takes place at about 4 months of age, give or take.
I've never weighed Truman, but he got him at five months old and he has definitely filled out. While he did't get "bigger," he definitely has gained in bulk. He had sort of a more babyish look too which I didn't realize until he lost it.
Quakers are full grown after the first molt or so. they def vary in size.. my Saphira is 84grams... while my Lily ranges between 113 and 118grams.. there are people on here with 140+gram quakers.