Height above sea level depends entirely on location. The lowest non-oceanic, nonsubterranean point on the Earth is on the shore of the Dead Sea, which is 1,391 feet below sea level; the highest point on the planet is at the summit of Mount Everest, 29,029 feet above sea level.
Between these two extremes, the average height above sea level of land on the planet is 2,756 feet. Sea level itself is the average of the high-tide and low-tide measurements of water levels at a particular location on one of the world's oceans. This can be measured eustatically or from some fixed point such as the center of the planet.