Keeping in mind, of course, that a large pond can visually dominate your yard, ultimately pond size is largely a matter of taste. Even the tiniest pond will enhance the atmosphere, and sometimes a number of small water features scattered throughout a large property can be more effective than one massive one. Some may prefer to design their entire yard around an extensive boardwalked pond or meandering stream. There are just a couple of things to consider:

The size of a formal fountain (particularly the height) when in sight of a house, should be proportional to the size of the house. A 4m (12') tall fountain in front of a single-story house can sometimes create an oddly distorted sense of perspective.

The depth needed for a water feature will depend primarily on the types of aquatic life which will call it home. Most hardy water lilies like 20-30cm (8"-12") of water above the rim of the pot, while some irises prefer up to 40cm (15") of water. The safety of pond fish can depend on their ability to retreat below the depth of predators. A steep-sided pond with 60cm (2') of depth should keep the raccoons at bay while a solid 90cm (3') is really needed to thwart the herons.

The cost of installing a finished pond is directly proportional to the size.