  In previous attempts to measure Omega_0 from the angular size-redshift
(theta-z) relation of double-lobed radio sources, the observed data have been
consistent with a static Euclidean universe, rather than with Friedmann models,
and past authors have disagreed as to what effects are responsible. These
results may be due to a variety of selection effects, different sample
definitions, and inconsistencies in the analysis undermining the results. Using
the VLA FIRST survey, we investigate the theta-z relation for a new sample of
103 double-lobed quasars, carefully addressing the various potential problems
which, we believe, have compromised past work. Before focusing on cosmological
constraints, we investigate the possible impact of correlations among the
intrinsic properties of these sources over the entire assumed range of allowed
cosmological parameter values. For all cases, we find only mild apparent size
evolution which is found to arise mainly from a power-size correlation coupled
with a power-redshift correlation; intrinsic size evolution is consistent with
zero. We also find that in all cases, an evolution-free subsample can be
defined, whose theta-z relation should arise mainly from cosmological effects.
Contrary to past work, we find that the observed theta-z relation for our
sample is more consistent with standard Friedmann models than with a static
Euclidean universe. In particular, we find that a flat, matter-dominated
universe, a flat universe with a cosmological constant, and an open universe
all provide comparably good fits to the data, with the latter two models both
yielding Omega_0 \approx 0.35 with 1-sigma ranges including values between \sim
0.25 and 1.0; the evolution-free subsamples yield values of Omega_0 near unity,
though with even greater error ranges.
