We report new determinations of the radius and mass of the pulsating helium rich, hydrogen-deficient star LSS 3184 (BX Cir) using measurements of radial velocity and angular radius throughout its pulsation cycle. Measurements of radial velocity, and thus changes in stellar radius (\Delta R_\star), were made using Anglo-Australian Telescope echelle specta. Hubble Space Telescope ultraviolet spectra and ground-based BV photometry were used to find temperatures and fluxes throughout the pulsation cycle. The temperatures and fluxes were used to find the angular radius of the star (\alpha). The \alpha, \Delta \alpha, and \Delta R values thus found were used to calculate the mean stellar radius \langle R \star \rangle = 2.31 \pm 0.10 R_\odot. If we use the previously determined log g = 3.35 \pm 0.1 for LSS 3184 and our radius estimate, we find its mass to be M_\star = 0.42 \pm 0.12M_\odot.