  We find a linear relationship between the size of a massive star's
main-sequence bubble in a molecular environment and the star's initial mass:
R_b \approx 1.22M/Msun - 9.16 pc, assuming a constant interclump pressure.
Since stars in the mass range of 8 to 25-30 Msun will end their evolution in
the red supergiant phase without launching a Wolf-Rayet wind, the main-sequence
wind-blown bubbles are mainly responsible for the extent of molecular gas
cavities, while the effect of the photoionization is comparatively small. This
linear relation can thus be used to infer the masses of the massive star
progenitors of supernova remnants (SNRs) that are discovered to evolve in
molecular cavities, while few other means are available for inferring
properties of SNR progenitors. We have used this method to estimate the initial
masses of the progenitors of eight SNRs: Kes 69, Kes 75, Kes 78, 3C 396, 3C
397, HC 40, Vela, and RX J1713-3946.
