Depends on if you want it to be permanent or seasonal. YOu're going to need to make sure that you have at least a ft below the frost line for you fish to make it through the Winter. Depends on if you have water movement what you want fish load etc, both gold fish and koi get BIG.
hth
chirs
I might as well use this thread to mention I have several articles on pond keeping. In fact, on my website I have a whole page dedicated to aquaria related articles. This page is growing quickly, and the articles are becoming very popular if I say so myself.