Molecular dynamics simulations have been used to examine the effect of the size of a spherical inclusion in generic model polymer nanocomposite systems. Results are presented concerning the effect of the increasing particle size on the thickness of the interphase, i.e., the perturbed zone of polymer surrounding the inclusion. The behavior of the mass density, molecular orientation, fraction of trans conformers, as well as dynamic properties are presented as a function of the distance from the nanoparticle surface. The effect of temperature on these distributions is also discussed. Long simulations have been carried out to determine the variation in the glass transition of the filled polymers as compared to the pure systems. It is established that, within errors, the interphase thickness is independent of the size of the nanoparticle for the range of particle sizes analyzed. This information is particularly important for the second stage of the project where it is used in continuum micromechanical calculations to try to explain the behavior of the mechanical properties of the model nanocomposites. The confrontation between continuum and atomistic approaches will be the subject of a future publication.