The size and material of your fence post and footer depend on the type, height, and design of your fence, as well as the soil conditions and climate in your area. Generally, you should choose a fence post that is at least one-third as long as your fence height and at least 4 inches in diameter. You should also choose a concrete footer that is at least 3 times as wide as your post diameter and at least 6 inches deep. You should also consider the durability and resistance of your fence post and footer materials, such as wood, metal, vinyl, or composite.