Thanks for the good words Tom
Unfortunately Tom, there's no clear cut parameter on how big a koi pond must be. All I can tell you is this. If I was going to build a koi pond for a few koi, it would still be at least 4 feet deep. It would also be at least 8ft by 8ft. There are compromises that you would have to live with if you went much smaller on your design. If you made your pond any shallower than that, some of your problems would include predator invasion, rapid temp changes and a perpensity for suspended algae blooms. If you went much smaller in surface area, some of your problems could include oxygen transfer at the surface, rapid water chemistry changes, a high level of required maintenance and an overcrowded appearance to your pond.
If you go to "koijoy.com," you'll find all the supplies you could ever need to build your pond. Remember Tom....the more filtration you have per fish...the less maintenance you'll have to do. The more water there is in your pond per fish....the less maintenance you'll have to do.
If I can help with anything Tom, let me know.