There are several things to consider. First, koi ponds do better in a shaded area than in full sun. Full sun will work, but a shaded area will generally slow the algae growth down and will prevent the water from possibly getting too hot during the summer months.
If you choose to build in a sunny area, it might be practical to think of building some type of arbor, either now or in the future, to give your pond some shade. Unlike a swimming pool, leaves from trees near the pond should not create a problem if the pond is properly designed.*
In many areas local building codes require any pond deeper than 2 feet be within a fenced in area, which could present a problem if you want to build it in your front yard. Once you have chosen a general location for the pond, we recommend you use a garden hose to outline the exact location, size and shape of the pond. This can be extremely helpful in giving you a better idea of exactly how the pond will look. After placing the hose where you plan to build the pond, stand back and examine it from several locations and ask yourself. Will it be the size pond that I want? Will it be as visible as I would like it to be from, say, the back deck, the kitchen window or wherever I would like to be able to see it? Keep in mind, this pond will most likely be the central feature of your whole landscaping.
Once you have the general outline shape of the pond established you can get an approximate idea of how many gallons it would be by multiplying the (average length) x (average width) x (average depth) x (7.5 gallons per cubic foot).
*One note of caution: Pine trees are the one exception. Pine needles tend to create a real mess in a koi pond. Unlike leaves, needles can travel through the skimmer basket and the leaf basket on the and end up in your impeller. If possible, do not build a koi pond near a pine tree. Redwood trees, although messy, are not near the problem pine trees are.