Polymer nanocomposites are promising materials for dielectric waveguides in high-data-rate communications, where extremely low loss is required. In this paper, we study the effect of titania (TiO2) nanoparticle size (30-300 nm) and concentration on the effective permittivity (epsilon (eff)) and dielectric loss (tan delta) of polypropylene (PP) nanocomposites in two different frequency ranges: 100 Hz-300 kHz and 140 GHz-220 GHz. To aid the dispersion of TiO2 in the PP matrix, polypropylene-graft-maleic anhydride (PP-g-MA) is added. Using this approach, an epsilon (eff) of 6.84 with tan delta of 0.0049 at 220 GHz is achieved in a 21.5 vol% 100 nm TiO2/PP nanocomposite. We find that epsilon (eff) is insensitive to nanoparticle size in both frequency ranges while tan delta appears to depend on the filler size at the low frequency range. By using complex permittivity in Lichtenecker's model, we are able to separate the loss contribution of the polymer matrix from that of the TiO2 nanoparticles. Our results provide insight into the choice of nanoparticle size and the effects of compatibilizer on millimeter-wave dielectric properties.