  The jet opening angle and inclination of GW170817 -- the first detected
binary neutron star merger -- were vital to understand its energetics, relation
to short gamma-ray bursts, and refinement of the standard siren-based
determination of the Hubble constant, $H_0$. These basic quantities were
determined through a combination of the radio lightcurve and Very Long Baseline
Interferometry (VLBI) measurements of proper motion. In this paper we discuss
and quantify the prospects for the use of radio VLBI observations and
observations of scintillation-induced variability to measure the source size
and proper motion of merger afterglows, and thereby infer properties of the
merger including inclination angle, opening angle and energetics. We show that
these techniques are complementary as they probe different parts of the
circum-merger density/inclination angle parameter space and different periods
of the temporal evolution of the afterglow. We also find that while VLBI
observations will be limited to the very closest events it will be possible to
detect scintillation for a large fraction of events beyond the range of current
gravitational wave detectors. Scintillation will also be detectable with next
generation telescopes such as the Square Kilometre Array, 2000 antenna Deep
Synoptic Array and the next generation Very Large Array, for a large fraction
of events detected with third generation gravitational wave detectors. Finally,
we discuss prospects for the measurement of the $H_0$ with VLBI observations of
neutron star mergers and compare this technique to other standard siren
methods.
