is it true that if you leave a goldfish alone in the fish tank, it will grow extremely big

It matters on how large the tank is and how clean the tank is. The goldfish basically won't grow large if you don't have good water quality or a large tank. If you put lots of fish in the tank, the tank will just get overcrowded and the fish won't grow as quickly. If you have a pond, it will work much better, but the maintenance is bad.

Here is a little thing to make your tank clean:

Water quality is usually the main problem when a fish's condition

changes abruptly. The next thing to do is make sure you have the highest water quality possible. This would mean 0 ppm of ammonia, nitrites, and nitrates; all of which you can test using a test kit or by having your local pet store test them for you. Don't add any more fish until you've had your water tested. If any of these levels are too high, you should do an immediate 25% water change. If your levels of ammonia are high, three 25% water changes over the course of a week may help.

In the future, you should perform a 25% water change every two weeks. This will help to control levels of ammonia, nitrites, and nitrates. The water changes will reduce any problems with water quality and help the fish combat parasites plus reduce any harmful bacteria.

In order to maintain low levels of these toxins, you should also make sure not to overfeed your fish or overstock your tank.

Overfeeding of your fish can increase these levels in the water as the uneaten food is allowed to decompose. At each feeding you should feed no more than what your fish can eat in ONE minute, while allowing a minimum amount of food to fall to the gravel. When it comes to feeding fish, less is more.