Proper Pond SizeProper size – The size of the pond is very important depending on what you are trying to accomplish. If all you want to do is take 3 or 4 small koi and grow them up to 16″ long, than the pond size is not that important. You can easily do that in a 1500 gallon pond. If you are looking to grow those same koi up to 30″ than you will need a much bigger pond. To grow to that size koi need room to swim and exercise. Realistically, you would need at least a 6000 gallon pond.