Goldfish require a large swimming space and lots of filtration. If you have a standard, long-body goldfish, such as a comet, sarasa or shubunkin, your filtration should be slightly larger than what is recommended for the tank size.
Minimum Tank Size: For one goldfish, the minimum recommended tank size is 20 to 30 U.S. gallons (75.7 to 113 liters). Goldfish grow quite large and need ample space to live comfortably.
Aquarium Fish. The Goldfish Section. What Size Tank for Goldfish?Tank size is a subjective decision for all fish keepers. There are some well-respected online databases and sources of fish information which we recommend.

Furthermore, visual representations like the one above help us fully grasp the concept of Tank Size For Goldfish Growth And Space.
In the wild, goldfish can naturally grow to between 6 and 16 inches long. For optimum health and growth, a single goldfish requires a minimum 20- to 30-gallon tank. Each additional goldfish requires a similar amount of space.
Tank Size: Goldfish grow rapidly and need space. Small aquariums lead to stunted growth and poor health. Start with a minimum of 20-30 gallons for a single fancy goldfish and at least 50 gallons for long-bodied types like comets.
Does a small tank size inhibit goldfish growth? Learn all about goldfish tank size with this detailed ([year]) guide...
Goldfish Size & Growth Chart. Below are some typical numbers for most types of goldfish. Its important to remember that they can grow faster and larger, depending on several factors, including their species, tank or pond size, water quality, etc.