The concept implies the presence of rooms with the same area for girls. It is important that each child is equal to the other. If you make rooms of different sizes, this is not a very good attitude towards one of the girls.
Also, space between the existing bathroom and one of the bedrooms. You can make there the reading nook. This can become a very cozy space for girls. They could read their favorite books together, covered with blankets, enjoying the sun from the window.