In order to give an absolute definition of size for a fundamental theory of matter, we have to assume then that there is a limit to how finely we can make observations, and how negligible the subsequent disturbance can be.
A limit which is inherent in the nature of things and can never be surpassed by improved technique or increased skill on the part of the observer.
If the observed object is not affected by the limiting disturbance, it is big, and we can consider it classically, however if the disturbance is not negligible, then we require a new theory for dealing with it.