It is ok to use one bigger than you think you may need. If you have Koi, you may have special filtering needs. This is one of the most important things to do. The gallons affect the size of pump and filter you will have, the amount of fish to put in, and the dosage of chemicals like dechlorinator and salt. Multiply the average width by the average length by the average depth, then multiply that by 7.5.
Most people think they have way more gallons than they do.
If you can, monitor your water meter while filling the pond for an accurate measurement. If you are on a municipal water supply, then chlorine has been added to the water which must be neutralized with dechlorinator. You should test the water from your tap for other noxious additives like chloramine, ammonia and for the levels of hardness and pH. Adjustments may need to be made if the water is not properly balanced. Most aquarium stores can check your water for you. We don't advocate the use of chemicals beyond those needed to correct the water quality from the tap.