One of the most common mistakes people make with their first pond is that they build it too small. The smallest pond we recommend is 1500 gallons, which would be approximately 7 feet wide by 8 feet long and 4 feet deep.
That may sound huge to many beginners, but most people who already have a koi pond will agree that if you build a pond much smaller than that, you will not be happy with it. The problem is that koi are extremely addictive and it is hard to be content with just a few. Also, keep in mind that you may start out with several 6 inch long koi, but within 3 or 4 years they could grow to 24 inches or more. Most people, after a while, wish they had made their first pond bigger. On the other hand, you can also make your pond too big. Bigger is not always better.
The largest pond we recommend building is 15,000 gallons, which would be approximately 17 feet wide by 21 feet long and 6 feet deep. If you build a pond much larger than that you run into several problems. In a very large pond it can become almost impossible to catch the fish if you need to. It can also be very expensive to medicate the pond if that becomes necessary. Additionally, all the water in the pond will need to be filtered at least once every three hours. Running pumps large enough to accomplish this can cost several hundreds of dollars per month.
Finally, if you build the pond too large you may not enjoy the koi as much, because in a very large pond the koi are not always visible.