Nursery ponds can come in any size -- you'll want to choose this based on the amount of space that you have, how many fry you want to raise at a time, and how you intend to treat the water within. 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.