Of course, some humans are larger or smaller than average, but you can still account for that to get a decent distance estimate. And the overall trend is crystal clear: the farther off an object is, the smaller it appears. The trend is so obvious, in fact, that we can see the rate of change is linear: Double the distance, and the object will appear to be half its previous size. Look at it 10 times farther away, and it will seem to be one tenth as big.