I’m bit confused with the formula.
In my understanding, it should be:
rTexture = SomeConstant * atan(size/distance) # not tan(size/distance)
and SomeConstant is proportional to the number of pixels in 1 radian ( or degree) of the camera (or eyes).
With this formula, estimation error getting larger when (size/distance) ratio getting large, because the distance to the object cannot be represented with a single value. So, some function may be required for larger (size/distance) case.
Is it tan(x) presented in the lecture? I can not agree with it. Because
(Size/Distance) in range [Pi/2:Pi] results in negative value. The case with Size 4 of a car from Distance 1 is just a lucky case without seeing a broken result. Please check a result with a car sized 3 m or 1.57 m.