with prospective using the size of known objects to determine the size of other objects can be deceiving. A parallax photo taken with parallax camera corrects certain measuring issues with perspective.
The point is that if you know an objects dimensions such as a door width; that distance must be reduced along the length of another object away from the perspective and increased as you move closer. It's subtle determination that is helped if there is another similar known object dimension further way in the perspective.
