A few pieces of advice.

1) The fish growing to the size of their tank theory is a myth. If the fish don't grow much after a few years it is usually due to stunting, which is usually caused by poor water quality.

2) Goldfish need a large tank with a good filtration system, the fancy varieties need at least 40 litres per fish and the common varieties need at least 80 litres or better still a pond. If you get a heater the tank would make a nice home for a small group of Microrasboras and some Amano shrimp.

3) Always cycle your tank without fish, if you use fish to cycle the tank they will be exposed to the high levels of ammonia and nitrite produced during the cycling process. Provide a source of ammonia to kick-start the cycling process by using another source of ammonia such as fish food or bottled household ammonia from a supermarket or chemist. To monitor the cycle you will need a test kit (available from an aquatic store), only add the fish to the tank once the ammonia and nitrite both read 0ppm on the test kit.