Koi fish can get up to 2 feet long or more with the right space care and conditions. They are only suited for a very large aquarium of 500 gallons or more. But they can be kept briefly as juveniles in a smaller tank. They should be kept in a pond because they get so large.
When you go tot he fish store to get it you need to tell them what size tank you have and they will show you what size koi that you need to put in there but I know the more that you have the smaller they have to be.