Experiments recently showed that the finite lengths of single-wall carbon nanotubes (SWNTs) randomly dispersed and randomly aligned in a composite material are responsible for the appearance of a broad peak in its terahertz conductivity. We investigated, both theoretically and experimentally, the influences of the cross-sectional diameter and the acid-induced p-type doping of SWNT bundles in composite materials on their terahertz conductivity peaks (TCPs). We found that the TCP blue-shifts if the inclusion diameter is larger, and that doping enhances the effective conductivity of the composite material. But, a theoretical prediction of the blue-shifting of the TCP by p-type doping was only weakly supported by experimental evidence. All experimental observations turned out to be in good qualitative agreement with the concept of localized plasmon resonance in SWNTs.