Koi are very large fish, so they need a lot more space compared to smaller pond fish. Those looking to install a koi pond will need considerable space to do so. An ideal size for a koi pond is around 25 feet long by 13 feet wide, and at least 3 feet deep. Smaller koi ponds can be as little as 8 feet long by 6 feet wide, but the koi may start to outgrow it over time.