If you were to lay the girls out on the floor, that room is, like, at least 2 and a half girls long and 2 girls wide. A pretty big room I think.
You have a twin bed? You could lay 9 mattresses on the floor,
Growing up I had the big bedroom, 10 x 12, only 6 mattresses would fit.
That's why people have measuring tapes. You can also use a yardstick, or a 12 inch ruler to measure the size of your future room. Lay the ruler on the floor and start counting!
Don't worry about the size of the room. Just be thankful it will be yours alone.
Is a 15 by 12 foot room big enough for a 14 year old girl? I'm moving. I wanted to know, and I don't know how big that is mathematically sooo... yeah.