Mine(which were in a proper sized area) went from 2" to 8" in a little over 7months.. they were also fed alot of good foods, like mango, lettuce, Hikari goldfish foods, oranges, grapefruit and other veggies and fruits.
I highly suggest you do NOT do this.. A min. size pond/tank for 4-6 koi would be around 2,000 gallons... or more.
thanks, I guess Ill have to go to petco soon (maybe tomorrow) and get something there, my fave LFS dosent carry anything but show sized fancies and koi.
I would build the pond or whatever size tank (of a minimum of 1000 gallons + 100 gallons every added koi) and then get the koi, not the other way around