  We probe the environmental properties of X-ray supernova remnants (SNRs) at
various points along their evolutionary journey, especially the S-T phase, and
their conformance with theoretically derived models of SNR evolution. The
remnant size is used as a proxy for the age of the remnant. Our data set
includes 34 Milky Way, 59 Large Magellanic Cloud (LMC), and 5 Small Magellanic
Cloud (SMC) SNRs. We select remnants that have been definitively typed as
either core-collapse (CC) or Type Ia supernovae, with well-defined size
estimates, and a thermal X-ray flux measured over the entire remnant. A catalog
of SNR size and X-ray luminosity is presented and plotted, with ambient density
and age estimates from the literature. Model remnants with a given density, in
the Sedov-Taylor (S-T) phase, are overplotted on the diameter-vs-luminosity
plot, allowing the evolutionary state and physical properties of SNRs to be
compared to each other, and to theoretical models. We find that small, young
remnants are predominantly Type Ia remnants or high luminosity CCs, suggesting
that many CC SNRs are not detected until after they have emerged from the
progenitor's wind-blown bubble. An examination of the distribution of SNR
diameters in the Milky Way and LMC reveals that LMC SNRs must be evolving in an
ambient medium which is 30% as dense as that in the Milky Way. This is
consistent with ambient density estimates for the Galaxy and LMC.
