Decide based on how you plan to use your fence.
Corralling kids or pets: Three to 4 feet prevents wandering but is low enough to leave the view intact and to chat with neighbors.
Enclosing a pool: Code requires 4 feet, minimum. The maximum space between pickets is 4 inches.
Maintaining privacy: Six feet, maximum. Many communities require a permit and engineering plans for anything higher.
Controlling deer: Seven feet discourages casual browsers, although persistent bucks can clear 15 feet.