  We use high spatial resolution observations of CO to systematically measure
the resolved size-line width, luminosity-line width, luminosity-size, and the
mass-luminosity relations of Giant Molecular Clouds (GMCs) in a variety of
extragalactic systems. Although the data are heterogeneous we analyze them in a
consistent manner to remove the biases introduced by limited sensitivity and
resolution, thus obtaining reliable sizes, velocity dispersions, and
luminosities. We compare the results obtained in dwarf galaxies with those from
the Local Group spiral galaxies. We find that extragalactic GMC properties
measured across a wide range of environments are very much compatible with
those in the Galaxy. We use these results to investigate metallicity trends in
the cloud average column density and virial CO-to-H2 factor. We find that these
measurements do not accord with simple predictions from
photoionization-regulated star formation theory, although this could be due to
the fact that we do not sample small enough spatial scales or the full
gravitational potential of the molecular cloud. We also find that the virial
CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic, and
that the excursions do not show a measurable metallicity trend. We contrast
these results with estimates of molecular mass based on far-infrared
measurements obtained for the Small Magellanic Cloud, which systematically
yield larger masses, and interpret this discrepancy as arising from large H2
envelopes that surround the CO-bright cores. We conclude that GMCs identified
on the basis of their CO emission are a unique class of object that exhibit a
remarkably uniform set of properties from galaxy to galaxy (abridged).
