I dont think koi will be a good idea, but adult common/comet/shubunkin goldfish will be great in a pond of that size. koi can reach 36"+ (note the PLUS) I have personally seen koi much larger than 36". My rule of thumb for a koi is 1000G PER adult koi.
those goldfish which I listed reach 16-20" on average.
those goldfish which I listed reach 16-20" on average.