Yes, goldfish can grow to be 15 inches long or even larger. Tinkerbell is a cute name for now, but she will quickly outgrow that name. Goldilocks might work as she does have some gold in her fins.
My sister wants to grow coy in her backyard swimming pool, which she has convert into a fish pond. The pool is 9 feet deep in the deepest end and three inches deep in the shallow end. She put rocks and reeds in the shallow end, not knowing that tiny baby crayfish, only 1 mm long, were on the reeds hiding. Soon the entire bottom of the pool had black spots from a surprise snail bloom. They suddenly died leaving their white shells behind. Then the tiny crayfish started hiding in the shells, but quickly outgrew those hiding places. Now they are crawling all over the place.
She had always had mosquito fish ever since she discontinued swimming. These fish are now two to three inches long. Somehow, frogs found her pond, but these mosquito fish would eat the tadpoles nibbling away at their legs, so the tadpoles soon died and the crayfish finished them off.
She went to the pond supply nursery, and purchased water lilies and other water plants.
In her pond are also three species of dragonfly nymphs, which might have been hiding in the water plants. These nymphs emerge as dragonflies after about seven years and fill the air with a delightful hum. To control the dragonflies, a starling has taken up residency near her pond. It is there year round.
Just this year, she purchased some gold fish, small ones, but big enough to survive the hunt of the mosquito fish. Goldfish tend to be bottom feeders while mosquito fish stay at the top to eat mosquitoes and flies. She feeds the goldfish pellets that sink to the bottom, so these goldfish are not tame, but neither do they need to swim to the top and become prey to the mosquito fish. These goldfish hide in the plants when they hear her approach. However, the mosquito fish have learned to greet her in an attempt to get the first flakes of food. They follow her around the pool begging for food. Her friends who also have mosquito fish claim that mosquito fish usually hide to avoid human contact, but not these fellows! The first fish who come to greet her in the early twilight hours get the largest flakes of food and the first mosquitoes too. There are not that many mosquitoes left because they are attracted to water and quickly get eaten.
Mosquito she fly high,
Mosquito she fly low,
Mosquito light on me, (slap)
Mosquito fly no mo.
Enjoy your new pet, but realize that these goldfish ultimately need a huge outdoor pond where they will proliferate if the weather is temperate.