mine is 4 feet and wish I could have gone to 5 feet ,but since I'm only 5 foot 2 inches tall I would have had to stay in the pond with the fish or figure a way out LOL I like 4 feet but still wish I had gone slightly larger ,I can since I have extra liner rolled up on the sides ,but just to lazy to try it .After last winter I did invest in a pond heater 750 watts at petmountain.com it was 30 dollars .I live near the NC border but still gets good and cold during January and February .Koi like deeper water it seems since they stay at the deepest end all winter .Koi also need more care more filtration and more aeration more more more when it comes to koi .I don't have a bottom drain as I have already seen several fail on ponds if they are not installed right and I do not have a skimmer since I have no trees with leaves or needles to fall or blow into the pond .