What size pond for koi carp?

I have one natural pond and im not sure what its volume is but its about 2 foot deep and 1 foot wide and 4 foot long containing 3 goldfish and 2 shubunkins i thinking about building a larger pond for koi and maybe some other fish as my current pond is not big enough for koi of any sort i was wondering what sort of dimentions you would reccomend for about 7-10 koi and a few other fish and any reccomendations for good fish to keep with koi