The effect of nanoparticles (NP) on chain dimensions in polymer melts has been the source of considerable theoretical and experimental controversy. We exploit our ability to ensure a spatially uniform dispersion of 13 nm silica NPs miscible in polystyrene melts, together with neutron scattering, x-ray scattering, and transmission electron microscopy, to show that there is no measurable change in the polymer size in miscible mixtures, regardless of the relative sizes of the chains and the nanoparticles, and for NP loadings as high as 32.7 vol%. Our results provide a firm basis from which to understand the properties of polymer nanocomposites.