What's a good size for a backyard pond?

Does it freeze where you are? Are you planning on leaving the fish in there year round?

I'd say you want some to be no less than 24 inches deep, and around 600 gallon for a minimum. If you have hard, lasting freezes, you need to have it deep enough to avoid freezing (which can mean up to 6 foot deep in some areas), along with some aeration to keep an open area for gas exchange.

I am in Denver. Someone told me recently that three or four feet would work with an occasional heater. I was originally thinking 6.

He also told me there are about 7 gallons of water in a cubic foot.