This will depend on your available space and how resilient your intended stock fish is. Fry are very small, and typically are removed at about 3-4" regardless of the kind of fish. This means that the nursery stage doesn't typically require a large amount of space. While more room would allow for more fish, conditions are much more easily measured and controlled in a smaller environment.
If your nursery pond is exposed to the elements, orienting your pond with it's longest side parrallel to the strongest wind encourages mixing and contact between the water and air. Within a enclosed reservoir or tank, mixers will be used to accomplish this same concept.
