In article , Steve Willner
writes:

The hard part is determining
what physical size it corresponds to. This classical test has, due to
observational (not theoretical) difficulties not produced anything
useful up until now. (In some sense, CMB measurements are an
angular-size test, though.)

Why only "in some sense?" I thought they were exactly an angular
size test. Also baryon acoustic oscillations (BAO). So far, both
are consistent with standard cosmology. Aren't CMB fluctuations one
of the reasons the standard cosmology is standard?

Yes. By "in some sense" I mean that there are other parameters
involved. In other words, the physical length of the "standard rod" is
not known in advance, but depends on some other parameters.