Size does matter. Yes, the bigger the better. If you can afford a huge artificial pond in your backyard, the best is yet to come. For the rest of us, the aquarium is the best home for our goldfish.
Here are the parameters that determine the size of the tank you need:

The size and age of a goldfish
When you buy goldfish from a pet store, make sure they are young, so they too have a lifetime to grow. You can also start with a smaller aquarium. However, the obvious disadvantage is that you will need to upgrade to a larger tank as the goldfish continues to grow.

The potential size of a goldfish
There is a legend about the size of goldfish. Goldfish grow with the size of their environment. Indeed, if you put one goldfish in a tank and another goldfish in a pond, there will be far more goldfish in the pond than there are fish in the tank. However, this is not because goldfish grow in the environment. It’s because the environment, or the lack of an optimal environment, prevents the goldfish from growing. Therefore, you must take into account the potential size of the goldfish.
For example, a black Moorish goldfish can grow up to 6 inches naturally. Therefore, a 30 gallon /114 liter tank is best for keeping two goldfish. The average goldfish, however, can grow to more than a foot long. So you have to take that into account. The exact number of cylinder parameters may not be hard and fast, but many veteran goldfish enthusiasts believe that 4 times the potential size of the goldfish is the most appropriate length.

Types of goldfish and the number of goldfish
It goes without saying that the more goldfish you have, the bigger fish tanks you’ll need. Goldfish, on the other hand, can grow quite large over a period of years. So, don’t buy 7 goldfish on the first try. Be a miser.
Also, keep in mind that the thinner, ordinary goldfish grow larger than the round, fancy goldfish. Therefore, you will need a larger tank to keep regular goldfish.

Conclusion
After reading a truckload of information, you might not know, ‘So what size aquarium do I need?’ But if you still put a gun to my head and ask me the same question, I would suggest that if you want your goldfish to grow healthily, start with a 75-to 125-gallon tank. Keep the quantity to a minimum. Don’t keep all kinds of goldfish in an aquarium. Keep size to a maximum and quantity and variety to a minimum until you become more experienced.

The size of Common and Comet goldfish tank
For general goldfish and comet goldfish, we recommend tanks at least 4 feet long with a capacity of at least 30 gallons. Of course, this is the bigger the better! So more than that is also good.
If you plan to keep more than one goldfish in your tank, add 12 gallons for each additional fish.

Fancy goldfish tank size
For fancy goldfish, we recommend a tank at least 3 feet long and a capacity of at least 20 gallons.

If you plan to keep more than one goldfish in your tank, add 10 gallons for each additional goldfish.