So basically the further we are, the smaller an object appears and as we get closer to it, it enlarges progressively, is it so that the size of an object is always relative and that we can never measure the real size of an object. The Least Distance of Distinct Vision as far as I know is only the least distance at which the object should be positioned in front of our eye in order to be visible distinctly?
You're talking about how the perceived angular size of an object of fixed size changes with distance to the eye. In some sense, we never directly perceive the "real" linear size of an object with our eye; we merely know that it subtends some particular angle of our field of view, and angles are different measurements from distances.
To connect between angles and linear distances, we subconsciously use the fact that if two objects subtend the same angle in our field of view, and they are the same distance from our eye, then they have the same linear size. This is what allows us to hold a ruler next to a piece of string and say that it's 20 centimeters long.